Whats wrong with my gas trimmer?

I have a Craftsman 2 cycle gas weed trimmer. I normally have no problem starting it. If i shut it off after running it a while it will not restart. The next day it will start with no problems. This is a problem because when I use the trimmer to edge I go through string pretty fast. I do not like the idea of changing the string out while the motor is running. Whats wrong with my trimmer?