Of course, trying a clean nozzle would be the first thing to try. BTW, when
I start a new can, I usually use the nozzle from a can that's already in
use. I save the unused nozzles in my toolbox, so I always have a clean one
to got to when there's a problem.

I don't know why it works, but sometimes you can get the thing working by
taking off the nozzle and then putting it back. You'll get a little spurt,
and then nothing. If you repeat this a few times, often the thing will
start to work.