Not all cracks mean the same thing, and the pattern usually tells you the cause. Before assuming poor workmanship, work out which of three things you are looking at.
Fine hairline crazing across the surface
A network of very fine cracks, sometimes described as looking like crackle glaze, is shrinkage. The plaster lost its water faster than it could accommodate.
The usual cause is heat applied too early: a heater pointed at the wall, underfloor heating switched on, or a radiator run hard in the days after plastering. Direct sun on a south-facing wall in summer does it too.
This is largely cosmetic. Fine crazing fills with a mist coat and two top coats in most cases, or with a light skim of filler and a sand where it is more pronounced. It is a shared responsibility: the plasterer should advise against early heating, and the homeowner should follow that advice.
Straight cracks along a line
A crack running dead straight, particularly horizontally or vertically across a ceiling or wall, usually follows a plasterboard joint underneath.
Board joints must be reinforced with scrim tape before skimming. The tape bridges the gap between boards so that normal seasonal movement is spread rather than concentrated at the joint. Skip it, or use too little, and the joint telegraphs through as a crack within months.
This one is a workmanship issue. Boards move, that is expected, and scrim is the standard remedy. If cracks have appeared along every board line, the plasterer has a case to answer.
Cracks that reappear in the same place
If a crack has come back where one existed before the plastering, the plaster is reporting a problem rather than causing one.
Something behind is moving: settlement, a lintel deflecting, a joist flexing, thermal movement across a junction between old and new construction, or in older South London terraces the slow effects of clay subsoil shrinking and swelling with the seasons.
Replastering will not fix this. The new plaster will crack in the same line, usually within a year. The cause has to be diagnosed first, and if the crack is wide, tapering, or accompanied by doors that stick, that is a structural engineer's job rather than a plasterer's.
Cracks at corners and junctions
Fine cracks where a wall meets a ceiling, or at the junction of an extension with the original house, are common and usually benign. Different materials expand and contract at different rates and the junction is where that shows.
These are normally handled with a flexible decorator's caulk rather than plaster, since caulk moves with the joint instead of resisting it.
What to do about it
Wait until the wall is fully dry before judging. Some hairline cracking appears during drying and stops once the plaster has stabilised.
Photograph the cracks with a date, and if they are growing, mark the ends with a pencil line so you can see whether they are active. A crack that has not moved in six months is settled and can simply be filled.
Raise it with the plasterer while the job is recent. Most will come back and make good genuine shrinkage or scrim issues. Few will accept responsibility for structural movement, and they are right not to.