Hi there. If the cracks are along the ceiling line then it could be that the joists were still damp when boarded and skimmed (then drying and shrinking), or, no scrim was used where the ceiling joins the wall, or, (in the case of rendered walls) the ceilings were skimmed first and the sand & cement float coat then applied and skimmed. This would negate what the scrim is meant to do. If the cracks are running paralell to the ceiling But 2 or 3 inches lower, the render was applied directly onto the wall plate without mesh or plasterboard, or if plasterboard then the scrim was not fixed before skimming.
In answer to your question re fixing the problem - rake out the cracks with a trimming knife (V fashion), brush in a mixture of PVA and water (1:5), fill with a flexi filler and smooth off. You may need to do this twice as the first coat may shrink. Then sand smooth.