I've noticed that if anything else occupies the same block as an item frame, a comparator on the other side of the block can't see the item frame.

To reproduce, put an item frame on a block, and a comparator on the other side. If something is in the item frame, it should output a signal through the comparator. If something like a vine or a torch is put in the same block as the item frame (possible since frames are entities), the signal will shut off. If vines grow into the block, the signal will shut off when it gets updated.