Sounds like the shifter might not be engaging the arms for the shifter forks properly. 1st and 2nd share an arm, and maybe the pin on the bottom of the gearstick is sometimes swinging our past the notch it should sit in. It may be that the big spring thing that push the stick back to the centre (as in to 3rd/4th gear) isn't properly set up and letting you swing too far over toward 1st/2nd.
Hen