One of my neighbors explained that he was having a similar problem and worded it pretty much the same. When I went to check it out, I found that when he raised his hitch he was holding the lever in the raise position thinking that it would fall if he let it go.
Not sure if this is what your problem is, but since his explanation was almost identical to yours, I thought I'd put it out there. After the lift is all the way up,if yours is a model that doesn't kick out on it's own, you have to put the lever back in the center position.
My neighbor was using a borrowed tractor and I'm not sure how badly this could hurt it if he he had kept doing this, but I'm pretty sure it's probably not a good thing. So guys, if you are the type to be kind enough to lend your tractor to someone (I myself am not) don't assume that the borrower knows how to operate it. Even if he "used to run one all the time".