My wife and kids have this problem time to time, I on the other hand have had it happen a couple times.
In every case I found just the clutch adjustment corrected it. But all the suggestions made above such as adjusting cables and so on I do routinely, so they may be atributing to it.

I also found removing 1/2" from the shifters length (was in atv connections article about the lakota) made shifting for the wife and kids mutch easier. For the smaller footed poeple the shifter is way to long, but for me (11 E) I've had no problems.

I did try alot of oils way back last year, but I only made notes of how the clutch acted because I never had shifting problems. I run valvoline 20W50 automotive racing oil.