Two things it can be. THe poppet assy within the housing has a cracked or damaged seal or you may have a bad "o" ring under the bonnet assy. Both easy to fix. Shut off lower shut off valve. open test ports to relief any pressure inside of the unit. The poppet should drop down. Then unscrew the bonnet assy. Be careful, as it is only plastic and can break if you try to remove it with channel locks. Hopefully you will be able to get it off by hand. After that, just inspect the "o" ring under the bonnet for cracks etc and the poppet assy for the same. One other thing, it may just be a case of spiders building their nests in there and clogging things up.Reassemble with new parts as necessary.
close test ports and charge up.