Three possible causes:
- low damping oil;
- pushloc not adjusted right;
- moco damper busted.
Diagnosis
If your fork is low on oil you can tell by putting your foot on the wheel and pulling up on the bars. If you hear the fork suck air when its fully extended they there's not enough oil in the damper. If so, release all air pressure from the fork (neg first, then pos) and compress the fork. If fork doesn't compress fully, your damping oil has run into the lowers and you'll need to change the rebound rod o-rings. If it does compress fully, then drain oil from damper and refill with recommeded amount.
To adjust the pushloc, undo the grub screw on the spool, turn the pushloc on, rotate the spool by hand as far as you can so its in the locked position and then tighted the grub screw on the pushloc cable. It helps if you've got someone to give you a hand with this. If the pushloc is not pulling the spool to the full locked out position, the lockout will not work.
If none of the above works, you need to pull out the moco and examine for damage or breakage around the bottom where the lockout turns to close the lower ports. Also the lower o-ring may be gone, allowing oil to bypass the moco cartridge.