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