If it's cutting with the test cut button, then it would appear that the printer is capable of cutting. As a test, try the cut test at different points across the platen. If it cuts all the way across, then it's probably a file or RIP issue. If it doesn't cut all the way across, then you probably need a new cut carriage cable.
When you say the sheet cut button doesn't work - does it do nothing? or does the carriage go across, but it doesn't cut off the vinyl?
Be aware that many parts for that printer will be difficult if not impossible to find as Roland has stopped supporting it. Be careful about dumping too much money into it as you may not be able to get it working if the problem gets back to being a main board issue or something like that.