Look at your trough, if it can be filed as neophyte suggests, cool - otherwise get a shim.
Also - make sure the upkicker isn't blocking balls rolling from position 2 to position 1 (you have some degree of vertical adjustment). Consequently, a big enough trough divot can also stick a ball on the upkicker before it hits position 1 squarely.
Another way you'll know you're blocked in the trough will be by occasional phantom drains - when a stuck position 2 ball rolls to 1 while you have a ball in play (the vibration of a playing game can occasionally unstick balls).
EDIT: Of course, this could all be bad trough optos, as referenced by those above. The GLM kit is great. After buying it, the problem didn't go away for me - which lead me to the physical trough items i keep harping on.