Problems with demi glace

how long do you cook it? any tomatoes in it? is the mirepoix caramelized?

Your bones are getting roasted too much, or your tomato pince is getting burned, or your mirepoix is getting burned. Or when you degalze the roasting pan you are picking up too much burnt stuff. 

Even a small amount of burn on the bones and veg will turn a demi really bitter when you reduce it waaaay down--even if it doesn't look like much when you start. 

You can make a "white" veal stock by not roasting the bones, still adding tomato, and either caramelizing or not caramelizing the mirepoix. I say "white" because the appearance isn't really white, it just means you aren't roasting the bones. If you want/need a roasted flavor later, you can incorporate that into the small sauce later (like, if you are making a lamb sauce, take lamb bones, roast and caramelize them, then deglaze with veal stock and reduce). You can use both animal bones (pork, lamb, duck, etc) later in the sauce, or even meat scraps work as well for this (tenderloin trim, pork trim, etc). 

If you make a white stock, I recommend you blanch the bones at least once to get the really nasty stuff off of them first. This is optional, but you'll be skimming a lot more if you skip this step. 

 Crack bones first  Then I agree bring bones to a boil first then throw liquid away. Start again in cold water.  You are roasting your bones at to haigh a heat turn down oven and do more slowly when you roast turn the bones every once in a while.

Surprised, I don't find celery leaves bitter at all. I love putting them into chicken salad, the celery flavor it lends with the tenderness of the heart of the stalk are perfect to me. I wonder if they're like cilantro, some people think it tastes soapy while others have a completely different interpretation.

usually the outside leaves are bitter, the yellow/pale green ones from within the celery heart are not bitter. Those are the ones you should use if you are using celery leaves. 

Oh yes, the inner leaves were the ones I was talking about. Sorry, I misunderstood.

All good replies, I would agree that tomato puree/paste adds some balance to the finished veal stock, maybe you are roasting the bones too long like stated above (also add some beef trimmings to slightly fortify meaty taste).  I still practice making demi glace the way I was taught which is half brown veal stock and half brown or espagnole sauce together reduced to 3/4 or half volume.  But I also put garlic, red wine, thyme and balsamic vinegar to taste.