How can I make my old acrylic paints dissolve and flow freely again?
I suspect that you are encountering a problem with the clumping of additives or fillers, because in a closed tube it is rather impossible to have acrylic dry. You could try to regrind them with a heavy glass pigment grinder, which would probably help to disperse the clumps. Instead of water also try adding a bit of acrylic medium or binder.