This is fairly common with many motorcycles if you are talking about shifting from a higher gear into first after coming to a stop. If this happens, about all you can try is to reengage the clutch partway, allow the bike to roll forward a bit as you apply pressure to the gear lever and it might move into a lower gear. If you get stuck in third or fourth gear when your forward movement ends, it will usually be difficult to shift into first without first turning off the engine and then you should be able to shift into first or neutral. I believe this is due to the slight drag of the oil on the clutch plates that keeps a constant load on the clutch when it is disengaged. However, the best thing to do is to make sure that you downshift through the gears and into first before coming to a complete stop.