Try pumping the pedal a few times before attempting to put the trans into gear.
Sounds to me like the slave cylinder is bleeding off and not allowing you to put it into a gear. Pumping the pedal a few times will build the pressure back up and allow it to work.
If that doesn't work, then its probably time for a new clutch.
'87 YJ, 4.2L built and bored, 8" of lift, 8.8 rear, AX15/NP 231J, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...