It sounds like you have a leaking up-stroke cushion valve. (Not a down safety valve) You will need to disassemble the free-lift cylinder and replace the check valve in the piston. If it leaks you will get fluid build up on top of the piston, it takes more force to push it out the check valve than it does to raise the inner mast rails, that is why the second stage cylinders will fully extend before the free-lift cylinder has enough pressure to force fluid out from the top side. This problem happens under normal conditions if the operator makes quick partial lifts. Under these circumstances fully extending the mast corrects the problem. Since it happened again I believe you need to replace the internal check valve, you will also need a free-cylinder packing kit since you need to dis-assemble the cylinder. I have had a problem where the check valve retaining ring (snap-ring) broke, thus causing piston damage. I needed to replace the piston also.