Difficulty shifting into gear, manual 5 speed

I do not think that my clutch is slipping, but when I try to shift into gear I sometimes have to increase the revs or muscle the shifter into gear.
This has become much more noticeable recently to the point where today I almost couldn't shift at all.
Thanx in advance for the feedback

I replace the automatic transmission fluid in the 5 speed stick trans. and it improves the shifting almost always.