Are you using a 100 wafer? If you are it's likely that the starting
dimension of your feature is too small. Remember that KOH etches
along the 111 planes and forms a 57 (?) degree angle with the plane
of the wafer (100).
eg ---\ /----
\ /
\----/
Once the two sides meet, you won't be able to etch any further.
Good luck,
-Tariq