Are your bees bringing in pollen? Watch them in the morning. If they are bringing in pollen, they may just be using it as fast as they bring it in. They also will store it on frames other than the frame with the brood you are looking at. My observation hive had me concerned, when I looked at the exposed frame with lots of open brood, there was no pollen, I noted this and then turned the hive around and on the other side was lots of stored pollen. It may be you are not recognizing it when they cover it with a little honey. The honey protects it from breaking down. The important is are they bringing it in to the hive.