E46 m3 not always going into gear properly

Hi, I own a 2002 m3 Manual which I sometimes have trouble with the gears.
Sometimes when you try and select reverse the gear stick appears to go in correctly but as you release the clutch the gear box grinds and the stick jumps back, when I try and reselect the gear it crunches in also it crunches into gear a you change down gears into corners etc
Does any1 have any advice?
Do you think it's the box or just the linkages?
Any advice welcome thanks

1) It sometimes helps to let the clutch out in neutral before engaging reverse. Never force the shifter. If it does go in smoothly, put it back in neutral and let the clutch out for a moment.

2) Do you know how to properly downshift? If not, you may have worn sychronizers. If so, rebuild or replace the gearbox. If you properly downshift, you don't need synchros.