a little thing iv noticed seems to help when bleeding shocks is to spin the shock shaft as you cycle the piston during bleeding. it'll spin the piston little by little and help get trapped pockets out through the piston holes.

the only other reason you can't bleed them that i can think of is the o-rings/seals on the bottom of the shock are leaking somehow. those do go bad, shrink or well, get worn out. get damaged from dirt and water. get damaged from a damaged shock shaft. or if the shock shaft is bent it'll do weird things to the o-rings as the shock cycles.

and Brakleen does a really good job cleaning shock oil. you can pick up a can of that at pretty much any auto parts store.