Do you have the Anderson Kantleak 4 way valve? If you do it may be the Issue. I've replaced mine twice. Simple to pull apart and check.
Remove the black knob
Remove the set screw, bottom of brass ring (its hidden)
Remove Brass Ring
Reinstall Black knob, twist and pull out
DO Not use petroleum products to reinstall.
Below is link to purchase repair kit
http://www.fitzilla.com/Store/page188.html
Hope this helps