Hello and welcome to just answer.
The first thing that you need to check is that all the runners and under the seat are free from any obstuctions. Anything small can fall down inside the runners and cause the seat to stop. I would use a torch and have the seat in the highest positon that you can get it and hve a really good look around as things can hide themselves really well inside the runners.
Once you have done that please feel free to come back and ask for further help and I will be happy to try and help you further on this problem.