yuck!
--i would say start with quick connects, pull them off and put them back on. (one hose at a time going to FEL)
--next would be a check valve (ball) inside spool valve is sticking and not letting oil in/out pending on direction of spool valve is operated.
--linkages out of alignment for joystick/levers for FEL
--relief valve some how out of major adjustment.
--if you have 3rd function joystick/lever for FEL for say operating graple or snow blower for FEL, and you DIY, the third function electronic valve might be bad (grasping at straws here)
--operate both curl or uncurl for bucket and FEL arms at same time. to see if you can get oil to by pass from FEL arms to bucket cylinders.
at your own risk. disconnect only one hose going to FEL. and then try to let it raise up (with tractor off), toss hose you disconnected into bucket. if major spool valve failure FEL could come crashing down.
--if you have a joystick... if you can get one hose off FEL arms. swap for a curl hose, and try going through joystick to see if FEL will lower.
i would be cautious of using the "FEL arm lock" to hold the FEL as you disconnect a house. to easy for FEL to suddenly drop, bang, and then lock bounce out of position and let FEL fall rest of the way. figure something out to properly support it.
--FEL's can be rather heavy. and simple 2x4 from FEL arms down to ground, i doubt i would trust. something slip at FEL or at the ground and it all comes down.
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until you can figure out if it is valve or cylinder. by swapping hoses between FEL arms and bucket cylinders. you have to many options to choose from. all of which, generally you want the FEL down on the ground. so you can work safely and not have the FEL come crashing down on you.