I would bet your cable is binding. Take a look at how the cable is routed--sometimes the factory techs route these things around turns, even looping the extra cable in a coil. They work best when it's as straight a pull as possible. Maybe try some lube on the actual valve itself, too?
This is why I wanted nothing to do with cable actuated valves and went right to electric actuated ones. Push button, valve opens...push button, valve closes.
2015 XLR Hyperlite 30HFS5 (mods being performed regularly)
2009 Salem LA 292fkds (gone)
Nights- ('12)23 ('13)23 ('14)15 ('15)31 ('16)25
2014 Harley Davidson FLHX (XLR cargo)
2014 VW Jetta TDI (diesel sipper, camping support vehicle)
2011 Ram 2500 CC 4X4 CTD, B&W Companion (toy hauler hauler)