Two things come to mind.... several people on this Board have suggested washing the lint filter in soap & water, drying it before putting back in machine. Guess it's because the fabric softeners can leave a residue that builds up over time.
Secondly, you may have to buy one of those long brushes made for cleaning way down into the machine and/or disconnect the vent pipe at the back and cleaning it, also. The lint filter doesn't catch everything.
Good luck, hope this helps.