Blow compressed air through it.

Try one way if that doesn't work try the other.


We don't use that style but we use the Prima Techs with the hose and bottle strapped to our arms to vaccinate pigs.
It seems that once in a while something gets stuck in the check valve, I always figure it is a piece of rubber from the bottle top. 

They're somewhat meant to be throwaways but we've probably given 50,000 shots with the two were currently using.