How can I make my old acrylic paints dissolve and flow freely again? I have El Greco acrylic paints, purchased and opened some 1.5 years ago, of course, I open them only when I use them. But now I observe that water is not an efficient solvent for them any more. I don't manage to arrive at delicate, soft, liquid, flowing paint easily. Should I use some additives or some other solvent for my acrylics?

Of course, at the end I managed to liven up my paints but the flow is still not so smooth and liquid as expected.
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.