the handle has splines inside,that match the regulator.These may have worn off.loosen the screw in the handle and have a look
If this is the case you may be able to re dress the splines with a small triangular file. The handle is made of pot metal and tends to wear which may reveal itself as sluffing off into the grooves. It worked for one of mine anyway. (also hit the regulator grooves as well)
