Ooooh, I possibly know! Had the same problem with mine and managed to repair it myself.
Take off the top vent housing using a standard allen key and stick it to one side.
If you look downward about 6 inches you'll see the fan itself, this fan is operated by your dial on the dash. What can happen sometimes is that it simply gets stuck.
This can be for one of 3 reasons.
1) The motor has packed in. (Worse case scenario, you'll need to get a new little motor put in)
2) For some unknown reason it's jamming and sticking against the back section of your dash. (If this is the case you just need to shave off a little bit of the fin that's getting jammed.)
3) The little handles that the fan pivots on aren't lubed enough. (Spray some WD40 or the such onto both of the little handles)
First though, pop that vent housing on the top off and peek inside to see what's happening to that main blade when you move thw dial to aim at different places.
Mine too felt like it was stuck on my feet, so I popped the top vent housing off and realized that it was jamming against the dash for some mad reason. I'd push it down (As long as the dial was set to my face) and it would click back down, but if I aimed it back at the windscreen/feet area it would rise and lodge again,
I shaved a slight bit off and hey presto it now works brilliantly,