Landrover Discovery 2004 rear door jamming.

Hiya everyone. Im new to this site and could do with some advice please regarding my Landrover Discovery Landmark 2004. The rear door keeps jaming and wont release. The handle on the outside seems fine but wont release at all. The centre locking seems to be working OK ( sounds ok ) Then sometimes it will work again. I was out yesterday after thinking Id sorted it all and it jammed yet again with my dog in the rear inside a dog cage.grrrrrr.lol So I had to lower the back seats to get into it.Not easy task. So anyone any ideas please what I can do to solve or repair this problem. Cheers all..Mike