Madhav,

A good thing would be to first check for cleaniness of your develop
and rinse beakers. Also check with any temperature change with the
hot-plate (is it showing  accurate temperatures), also if the UV lamp
is getting used-up, its intensity tend to reduce. Check the intensity
of the lamp.

One of the abve or all of the above reasons may contribute to your probelm.

Check with the integrity of the Photo-resist (expiry date and stuff).

If everything seems alright, then increase the exposure time by couple
of seconds (depending on your feature size) and increase the
post-exposure bake temperature by 10-15 C for the same time and try
developing.

Hope this helps.

Suraj Patil