Some condo association owners refuse to pay assessment.

I live in Illinois and am on the board for our condo association, we took over a mess from the previous people, we owe several thousand dollars the heat and hot water are common in the assessments. We voted to have a special assessment to bring things up to date. We have 30 units 5 in foreclosure 16 people have paid or are making the arrangements, 9 have not responded we spoke to some of them who basically said they are not going to pay. What action can we take or who should we contact in our state?