the nuts in the frame could be loose or the bolts are loose but I assume you already checked that. You can take it to a shop in the frame nuts are spinning round and round, they should have a tool to tighten them. There are videos on youtube how to make a tool and do it yourself. My sons would not tighten on his ZX6 so I just cleaned the area around the nut and JBwelded the SOB in place. Worked like a charm.
fahza fahza can you heeeaar me!