if the fluid was low, as you raised the mast, it would start to shake and shutter as air is being pumped into the cylinder
if you dont see that, its a good bet the hyd fluid level is ok

as far as not lifting unless its tilted fully forward or backwards
it could be something is getting pinched as the mast moves
or maybe the opening of the relief valve as you deadhead the tilt has something to do with it in the valve

either way DO NOT ATTEMPT TO FIX THIS YOURSELF
is see youre a short time clark "user"
working on and testing the mast is very dangerous and should only be done by trained and experienced techs

please call in a reputable repair shop to diagnose this issue
i can make it but the 3 hour travel fee (eachway + tolls) might not be worth it lol

about 35$ in tolls roundtrip