interesting, how does it behave with engine off, feet off all pedals and gearbox hot? (could be a clutch drag issue?)
my old car used to get a bit notchy and difficult to engage when the clutch was on its way out or needed bleeding.
another test is when it wont go in, pop it in neutral, clutch out, rev the engine to about 2.5k then quickly clutch in and try again - that way everything in the box is rotating so it should slot in.
if you park on the flat and do that test with the handbrake off and the car moves a tiny amount when you put it into 1st, the clutch is the culprit.
If not then you could try a gearbox oil change to start with. i'll have a look under the bonnet and see where the bits and bobs are relating to the gearchange turret, theyre usually on top to the rear in car.