Help! One of my jacks won't retract or extend!

We had lots of problems with our 4600 jacks. Finally replaced both fronts with 4800 ‘s. They are a bit longer than the 4600’s and greater lifting capacity. Check with happijac for rebuilt vs new. When Gary was still there he helped us out. Don’t know who does his job now. If you can get the 4800’s it’s worth it