When mine did this I found that the wire that connects the control and
the valve has kinked and when you moved the handle it would just fold
the wire over instead of moving the valve.  A trip to the local parts
store revealed that this must be a problem on other vehicles too as they
had plenty of bare stiff wire that cost me pennys.  The valve is easy to
get to and it's right above the spare tire.  I took mine out and worked
it until it moved freely.  With a torch I heated one end of the wire so
that it would bend and fed the new wire through the control tube. 
Figured out the length needed by moving the valve and marking where it
lined up with the control lever, wrapped the other end in a wet rag and
heated and bent it.  Installed, worked , happy...  In the interum, you
can just lower the spare and manually turn off the valve.  Get you
through the summer...
Mark