I'm Based in the UK

I bought a raptor 660 X reg (UK) It had a bad second gear as it was slipping really bad. I took the engine apart right down to the middle casings and looked at the gears to see if there was anything wrong with them and found that the gears were absolutely fine and nothing was rounded off or any teeth missing, I did find that the Shift forks had been grinding on something and I replaced them with new ones. I rebuilt the engine and started it up yesterday to find that the problem is nowhere near as bad but now and then something does slip ? Could it still be the gears or could it be something else.

I read the forums and replaced what was needed but I'm now stuck with no idea what else to check ?

..... Help.... haha!