Dead bluetit chicks

Yes! At first they were getting little spiders and caterpillars. (I have a camera box) Then the parents started to feed them from the Buggy Bit feeder. This worried me because the bits are quite hard and are not as juicy as the caterpillars (dehydration worry). When the first one died I had a look inside. All seemed well but the stomach; this seemed very hard and much larger than any other organ. when I opened the stomach it was full of a lot of what looked like chaff and even a small stone. When I removed the buggy bit feeder the parents began to feed sunflower hearts. The box is next to a very large oak. ( I thought this would supply plenty of the right food) I even bought some maggots but not one was taken. we have been very successful with maggots when fostering abandoned babies. the parents seemed very diligent 11 eggs 2 fledged. What have I done wrong. Any insights gratefully accepted Thanks Sarah