Pete ML

Hi all, I recently purchased a gasgas txt 125 for my son. When I purchased the bike the clutch was stiff. I removed the plates cleaned lightly sanded and re-installed. Initially it seamed the problem was solved but after a hour or so of ring the clutch has started riding. I've tried adjust the adjustment screw but I cannot get the clutch to fully engage. I am considering changing the clutch plates but I thought if the clutch plates were too work the clutch would slip and not drag. Can anyone pleaee offer some advice? Many thanks Pete