Hi Darren
I have found that sometimes a caravan leg if too enthusiastically wound back up jams in its recess - especially if things are a bit old and bent.
Plenty of WD40 sprayed and left for a good few hours then carefully turn the screw as well as teasing the leg down using a large screwdriver as a lever has worked for me.
It is also worth getting down on you back and laying under it to see if the leg housing has been knocked out of shape and is fouling the leg. Easily done and again you can use a large screwdriver to gently ease the housing back to shape so that the leg can be released.
That said - new ones are fairly inexpensive and are easy to fit. BUT - do make sure you get one that matches your caravan as when we had a new one fitted, when you put the "brace" on it, it fouled the bottom awning rail. Not a huge problem but one that can be avoided if you get a new "leg" with taller "feet".
Does that make sense? Time for a cup of tea I think!!