[honest bob]

The gears just don't line up sometimes as they come to a stop so you can't get them to mesh when you try to shift. Letting the clutch out (with the motor running) as Mrzappo says, will start the gears moving relative to each other and give them a chance to mesh. You can also move the bike slightly which lets the rear wheel move the gears so they can mesh. Try to shift to 1st just before you stop when you are out on the road, holding the clutch in at a stop does no harm (to the bike) and you are ready to move in an emergency.:jawdrop:
If it is still not shifting using these methods then you need to adjust the clutch per the manual.