Problems shifting into 1st gear

[Pete7874]

Every now and then (about every 3rd or 4th time I do it), the transmission will not want to go into 1st gear. Say I'm coming to a stop and I'm downshifting from a higher gear (like 3rd or 4th or 5th) and keep pressing the gear level down, it will get stuck at 2nd and not wanting to go into 1st. What I usually do at this point is give it some gas and let off the clutch a little to get the bike moving a bit. Once that's done, then I'm able to hit the gear level again and it'll finally go into 1st.

Any suggestions on how to fix this? About 4400 miles on the bike right now.

Seems like the problem became more common after I drained some excess oil from the engine (it was overfilled), but I don't see how this could be related... Oil level is at the full mark right now.