Without your recipe and a description of your technique, my "diagnosis' is going to be provisional. So, if you can get back with some more information, it would be appreciated.
For those who don't know (and they are legion): "Amish Friendship Bread" falls in the netherworld between "quick bread," "cake" and "yeast bread." It's usually partially leavened with a sourdough type starter that's more sweet and sour than truly sour because it's fed with flour, sugar and milk rather than flour and water; and partially leavened with a mix of baking powder and baking soda. Pudding mix and oil are common components. The "bread" is made with a batter rather than a dough. And, other than the normal feeding cycle which occurs before mixing, the mixed batter is not "proofed," i.e., allowed to rise outside the oven.
Figuring out your particular problem is more about cake than bread. The most common causes of cake falling during a bake are overbeating, bad baking powder (got damp, or very old), poor measuring, and an oven which doesn't hold its temperature. It's unlikely your starter is the problem, if it was the bread wouldn't have risen at all. However, it's easy enough to check.
If you're overbeating, knock it off. No. Really. How much should you beat? Just enough to combine the ingredients. You don't need to put air into the batter, the yeast and baking powder will take care of it for you. Combine, mix with a wooden spoon or rubber spatula (not an electric mixer), pour into your baking dish, and into a preheated oven pronto.
Why so fast? The baking powder and soda start working as soon as they're wet. After a few minutes, they start losing potential.
When's the last time you replaced your baking powder? Well? The stuff loses its ability to work when it gets damp -- even from humidity -- and sometimes just from getting old. Use a fresh tube. Use double acting baking powder (like Calumet or Clabber Girl) instead of a single actioner like Rumford. Double actioners are more forgiving. Once you've got quickbread baking down, you can switch to single actioners, but for the time being ...
Measure your non-yeast leveners carefully. For a typical Amish Friendship Bread, you should be using about 1-1/2 tsp baking powder and another 1/2 tsp baking soda. If you're using significantly more or less, that could be the problem.
Overbeating, and one problem or another with baking powder are far more likely culprits for a mid-bake fall than any yeast problem. In batters anyway, yeast usually either works or not.
Is your starter alive? A week before using, remove your starter from the refrigerator, turn 1 cup of it into a large bowl. Feed with 1 cup flour, 1 cup sugar and 1 cup milk. Stir to combine. Cover the bowl with cling wrap and leave the starter on the counter or in some draft free place and check it in 24 hours. Discard any remaining starter, and thoroughly clean your starter jar.
After a day, the starter should be doubled, or show signs of having doubled and then fallen; should show signs of bubbles; and should be giving off a slightly sour, slightly beer like aroma. If so, your starter is "proofed" and is okay. If not, your starter is dead and you need a new one.
If the starter is okay, stir it, cover it and leave it. Continue stirring once a day to reincorporate any "hootch" (liquid) which may have formed. Two days before baking, feed it again. The day before baking, give it a stir.
On baking day, take one cup for your bread, reserve one cup for next time, and give away the rest (that's why it's Frienship).
One last possible problem -- unlikely though -- is that your starter has been colonized by wild yeast which gets extremely active just before being killed by high temperature. That is, it creates a lot of volume in the middle of your cake which the glutens in the flour cannot support. Just barely possible -- but if that's the problem, the solution is to replace the starter with one you know to be good.
When you bake, give your oven plenty of time to preheat. If you usually give it ten minutes, give it twenty. This sometimes help stabilize cranky ovens; although it's usually not an issue at the relatively low temperature at which Friendship Bread is baked (325F ish).
Hope this helps,