The nozzles on spray paint cans will clog pretty easily, and a clogged nozzle makes the whole thing useless. A bath in some mineral spirits or paint thinner will make that nozzle good as new.
Photo by Mike Mozart
Your spray paint will get backed up completely if you leave paint to dry on the inside and outside of the can's nozzle. Fortunately, this tip from This Old House can save you the purchase of a new can:
...pull off the nozzle and soak it overnight in a can of mineral spirits or paint thinner. Use a sewing needle or wire from a twist tie to clear away softened paint from the tiny tube at the nozzle bottom; be careful not to enlarge or distort the tube.
If the clog is really bad, you can stick the nozzle on a can of aerosol lubricant and fire short blasts until it finishes clearing up. If you want to prevent this from happening again, just hold the can upside down and spray until it's clear the next time you're done painting.
Unclogging Spray Paint Nozzles [This Old House]