First, check the tension with a long piece of already spun yarn (commercial, acrylic, or something smooth). Increase the tension so the yarn will draw on readily, if it's scotch tension, tighten the scotch tightener; if it's double drive, increase the tension in the drive bands (usually the mother-of-all adjusts for this).
Once the drawing in tension is set okay, if there are still problems with draw-in on your handspinning, take a look at the diameter of the roving you've spun that is ready to go onto the wheel and check if it might be getting hung up in the orifice (if the yarn is too big around in spots to fit through there).
Also, if you are spinning dread-locky or fuzzy yarn, it might be getting hung up on the hooks from time to time as it tries to roll onto the bobbin. I've found that when this happens to me, I can't see the hangup, but when I try to remove the yarn from the hooks -- wa-la! I find a pesky few strands that have somehow wrapped themselves around one of the hooks. If this is so, then completely lift the yarn off the hooks, put it back on, and watch it as you roll it onto the bobbin to see if it is catching still or not.
(posted by me this day on spinning_camp)