I have a 2012 KX250F that up till now shifted pretty easily. Recently it seems shifting has become more difficult, like it's stiffer and I really have to pull up on the shifter to change gears. I tried making the clutch cable tighter show it pulled in more on the clutch but that really didn't seem to correct the problem. I always though clutch problems were more slippage related. Could this also be a clutch problem?