Do not grind it at a stretch - grind it in spurts of two to three seconds till done. In summer use ice cold water for grinding black gam. To grind rice, one cup of water would suffice. Grind it to rough or smooth paste. Try both ways and find the difference for yourself.
Fermentation makes or mars the idlies.
It is slow in winter and quick in summer. Most complaints that idlies not coming soft like sponges are due to lack of proper fermentation. The dough needs to ferment for eight hours at normal room temperature in summer. It takes longer in winter. The dough should become one and a half times.