Normal gas springs do not lock, but when not attached you have to overcome the gas pressure to move the piston. Try hooking them back to the bed and they should operate normally. (psi rating is marked on the strut)
I have 80psi struts on my cargo doors, very difficult to move when not attached..
Of course it may be possible for a strut to fail- locked open....Usually I have replaced mine for the opposite problem, no longer hold the door open.
Hooligan, Pensacola, Fl -U.S. Coast Guard 1956-1985
2016 Thor Siesta Sprinter 24ST diesel -1972 Moto Guzzi
2008 Suzuki Grand Vitara TOAD