Wow! I am sorry I didn't see this sooner. The larger Sonorencias tortillas, or sobaqueras (a bit larger), are a bit temperamental, but they are very doable for both home and restuarant use. It is hard to describe in words the exact process of making the giant tortillas/sobaqueras/sonorencias. However, the key things you will need is a soft, smooth and looser dough than normal (requiring a touch more water, touch more lard, and touch less baking powder); as well as technique to stretch and cook them appropriately.
Since I am not sure of the exact recipe you are currently using, here is a link to a tortilla article that I did for ChefTalk. The advice I will share in this thread is based off of this basic recipe: http://www.cheftalk.com/wiki/mexican-food-recipes-flour-tortillas
1) Use all purpose flour. Plain and simple. No need for bread flour at all as too much gluten development is likely part of your problem.
2) When making sobaqueras/sonorencias (i.e. the giant tortillas), you should be using little to no baking powder in the recipe. I personally use a little bit because without it, the tortillas of that size will tend to take on a texture-less appearance and mouthfeel (I hate that word, but that is the best way to describe it.) Plus, too much baking powder does result in a masa that's difficult to work with. I don't know the scientific explanation. I just know that the more BP I use, the tougher and more elastic the tortillas become.
3) Based on the recipe included in my article, for regular sized tortillas, you would use 1/4 cup lard. I would up this amount just a touch. When I make my 15-17" tortillas, I usually use a bit more than a 1/4 cup but no where near 1/2 cup; maybe use an addition tablespoon or two only at first. The additional lard give the masa a smoother feel, softer texture, and will make the masa a bit more easy to work with.
4) You will need to UP your water by a lot. The recipe in the article for 9" tortillas calls for 1 cup of warm water plus a bit additional to incorporate the remaining dry ingredients. For sobaqueras/sonorencias, you may be nearing the 2 full cups mark for water. Since this is for restaurant use and you want don't want to waste too much dough while perfecting the recipe, I would recommend you start with 1 1/2 cups very warm water, and incorporate as quick as you can and try not to go much beyond that amount until you have experimented a few times to know the proper feel of the masa.
5) for 9 " tortillas, you will get between 12-15 tortillas using the 3 cup flour recipe in the article. For 13-15" tortillas, divide the dough into only 8 equal pieces.
6) with the increased water and increased lard, you will have a very smooth and soft dough. It shouldn't be too sticky but may have some tackiness. When you divide the masa and coat the pieces with lard, set them out individually on a dusted table/cutting surface COVERED. Doing so prevents the divided balls from reforming into a single large mass if they were sitting in a bowl together. Then, rest them for at least 20 minutes but 30-1hr is great too.
7) Because this dough is so much softer than normal, dust your rolling pin and rolling surface well before rolling. You may need to continue dusting after every few strokes and/or when you flip the tortilla that you are rolling.
8) you should be able to roll the tortillas easily to 10"+ . Therefore, roll it out to this size then pull it off your rolling surface to stretch it the rest of the way to 15". This is where it gets tricky since you don't stretch it as you would an average tortilla. As with average tortillas, you will gently pull the edges of the tortillas as it sits on your palm/arm. But it will soon become too big for regular stretching, AND you need to move quickly so that it doesn't OVER-stretch and become too thin in the center (leading to uneven cooking.) So you need to do a series of flips/passes between your two arms to complete the stretching process--basically you are just transferring the tortilla from one arm to the back of the other. It is a little hard to describe so I will try my best here. Let's say you are starting with rolled-out 10" tortilla draped over your left side (forearm and palm facing up.) To transfer it to the back of you right arm and hand, sweep your right arm UNDER your left arm, catching the longest edge of the tortilla on the back of your mid-forearm of your right arm. Gently roll your your left arm towards you to help get the tortilla off of your left arm, while simultaneously rolling your right arm away from you so as to pick up the slack of the tortilla. You should now end up with the tortilla draped over your right palm and forearm. Reposition/center the tortilla on your right arm, and now repeat the process back to the left arm. Do this a couple of times to get the size you desire. It is this step that is the most temperamental since the center of the tortilla will thin out QUICKLY. With some practice, you will see which edges of the tortilla need to be stretched most. You may even be able to do the whole thing without the series of arm passes.
9) If you are cooking and serving the tortillas fresh, then cook them as you normally would making sure to cook thoroughly. Once the masa is made and divided, rolling and cooking a tortilla to order should take 5 minutes max. So if you only serve a few each night, roll and cook them to order. If, however, you are slammed with tortilla orders, you may need to pre-cook a batch for each night's service. But if you go this route, AND you plan to reheat when firing the order, you should slightly under-cook them since tortillas of this size can overcook and become crispy very easily.
I notice you are in Orange County. I am local to you and can stop in ANY TIME to demonstrate. Send me a pvt. message if interested.