String trimmers

I need a string trimmer clue. I'm talking about those Black & Decker electric thingums with a motor and a spool of plastic thread at the end of a handle, used for trimming the edges of the lawn. I had one that was a coupla years old, and it had a problem: instead of feeding more string when I bumped it on the ground, the string would cut off and no more would feed. I could unplug it, re-feed the string, and continue on for a couple feet more, before repeating the whole process. So I figured it was broken, and I bought another one. Straight out of the box, with the spool factory-mounted (so I'm pretty sure it's not on there wrong), I have exactly the same problem with the new device.

Help me o Intarweb!