Do you let the wafer sit for some time after exposure and then perform a
post exposure bake?
I have seen this problem usually with thick resists. The post exposure
rehydration and post exposure bake step gets rid of it. If I leave these
steps the deeloping seems to stop at a specific thickness and does not
process much even if you overexpose the resist or overdevelop it.

With Warm regards,

Mahesh Pitchumani