I agree with the others, you might be shifting too early and so the RPM will be too low for 2nd. Did you have trouble at higher RPM to shift into 2nd, like the gear shift would not move it, so you started trying to do so sooner? If so it may be that you were trying to shift too quickly (the actual action of shifting). It's okay to push the clutch in, go into N briefly, and let the RPM drop a bit before shifting into second. So say you start the shift at 3500RPM, maybe let it drop to 2700, every car is different though. A mechanical explanation might be that your throw-out bearing is shot, but I think you would hear or feel it if it was that bad.