There are a number of things that could go wrong. I'd suggest you show us the relevant code fragments in your JSP and form bean. Are the data in all the other controls being display ok, and the checkbox is the only one that isn't?
Even i faced this problem in my application What i did was i send the value as hidden property when it is checked And at <html:checkbox > we should apecify the value attribute as true That is when we check it shud return true. Once we checked and set the value and redisplay the form now it is showing checked in my appl. Its better if u send u r code. Then we can find what's the problem is