Possible clutch issues.

Hi there,
I have a Leon mk2 TFSI running 310 bhp I brought it as it is running 310 brake and with a stage 2 clutch. I've had it coming onto three months now and I've only had this problem twice to where I can't put it into reverse gear I've rang up a few places but they aren't sure if it's a great box issue or a clutch issue. It's hard to describe the issue as it doesn't happen often but after a small run to the gym and back yesterday I came home and I couldn't put it into reverse to park up and first gear felt tight to put it into after half hour I came back to the car and went into reverse fine but at first the reverse was a complete no no I didn't force it in but when I was going to push it in was like it was just a deadend. I've had new gearbox oil put in "70 80w" redline gearbox oil. gear changes are allot smoother but didn't resolve the issue any help on this will be much appreciated guys.