Hey fella, unscrew the back cap back of the bolt and take out the inner, you will come to a brass bit ... Lube this !! as this is very often overlooked and when it goes dry the symptoms your talking about usually happens, what you have there is a fantastic marker and to be honest very hard to beat. talk on here what your doing and maybe it`ll be sorted before you know it.
It`s one of the easiest guns if not THE easiest to maintain.
after you`ve done greasing that part , put the back together again and screw all the way in, but only to the point of you feeling it`s there !! don`t over do it !! then unscrew HALF - 3 x quarter turn this should put you at around 290 fps, we`ll go from there ???
bloomin heck mate if we can`t fix this for you I`ll have it off you at that silly money.