my leg wont wind down on my caravan?

i was at a site this morning and moved from a waterlogged pitch to a slightly less waterlogged pitch.
when putting the rear leg down it started to take the weight but then suddenly dropped and it wound up but will not wind down!
as this was my first time with a touring caravan it has really annoyed me as i bet its easy if you know what your doing or have been faced with this before!
any help would be greatly appreciated!