Could be air in the system, too. If the fluid level was low at some point it could have allowed air to get in. You could bleed it by cracking a hydraulic line at the farthest jack and extending the jacks. Make sure someone is stationed near the reservoir to make sure it doesn't start sucking air again.
It could also be a collapsed hydraulic line. Sometimes they're hard to detect because the inside of the line collapses and the outside looks normal. Check for a pinched line. A hydraulic shop can check for flow at the cylinders and figure out if that's the problem.