How do you repair a hydrolic jack that fails to lift?

It may just need to have some oil added to it, they do lose some over time. There should be either a rubber plug or a screw in plug toward the top of the cylinder on the side. If that is not the problem, it has to be rebuilt, new seal put into it. If it is a name brand, rebuild kits are sometimes available. Some off brand or import ones just have to be replaced.