Patterning using photolithography

Hello,

I am trying to pattern photoresists on silicon and I am facing some
problems and I am hoping that someone might have some suggestions as
to how to get rid of them.The photoresist I am using is Shipley S1813
and developer Microposit 351. The processing conditions I am using are
as follows:
Spin speed 4500 rpm for 30s, softbake at 115 C for 1 min, UV(350-450
nm, 76 mW/cm2)exposure 4 s, developer (undiluted) time 40s.

1. The mask I use is a mesh with square holes 5 micron size and 7.5
micron bar size. After developing I see that only the edges of the
square are removed, while in the middle a sqare region of photoresist
is still left behind, though of lesser height than the original
photoresist thickness. increasing the developing time dissolves the
pattern.Can anyone tell me why this is happening and how I can get rid
of it.

2. After the developing I want to remove the photoresist completely
using acetone, but I am unable to do so, and I believe I should be
able to do it. Any reasons why this is not happening?

Thanks,
Supriya