Shifting Problem from 2nd to 3rd

I don't really know much if anything about the transmissions on these bikes. I have a 2002 CR 125 which is having an issue shifting from 2nd to 3rd. All of the other gears seam to be fine occasionally i'll also have an issue from 4th to 5th but I don't usually go in 5th gear very often so I can't say if it's an issue or not.

Basically 1 out of every 5 shifts works fine from 2nd to 3rd the other 4 out of 5 trys are hard to get in, I'll have too try to hit it up a few times before it goes in. Sometimes slowing down a little helps. Any ideas on what to look for? The oil in the case is fresh and the same stuff that I have used in the past so that shouldn't an issue.

Thanks
Stan

Thanks for the advice. I played with the shift lever a little yesterday hoping that adjusting it may help but it didn't seem to. Guess I'll have to take it apart. I'll probably wait for a few more rides since after spring I don't ride much again until fall.