As Les says the best bet is probably just to fill and sand the cracks. I suspect the issue is down to how well the surface was sealed before plastering over, depending on the absorbency of the surface being plastered onto a good few coats of diluted PVA may be needed before it stops soaking in. Even then there is not guarantee that it won't crack somewhere.
If you went back to the brick work why did you decide to go for render and skim? I'd have had it dry-lined, much less likely to crack, and should give a better finish generally.