Cracks in plaster

Tim is right. Unfortunately even the best plasterers get cracks sometimes. Even when the joints are scrimmed with tape. When a stud wall meets a solid wall are the worst. Any slight movement Wether it's expansion or shrinkage it causes cracks. I have used that to toupret and would also recommend it. Goes solid. Good sand down with a fine sand paper and then use a good quality paint.

Unfortunately yes this often happens with a plaster re-skim. Have you decorated the room? If so it may need painting again. If you use a good quality emulsion ( trade paint or valspar v700) these paints can 'heal' certain fissures but only very hairline ones. Anything wider needs filling. Id reccomend you look at the toupret range of fillers, they have one that adheres to painted surfaces and is very fine. You need a fine filling blade and push it into the crack, then finish it to a minimum thickness, sand smooth then paint

This is very normal, its to do with movement (shrinking and expanding) easi fill and a good quality emulsion should fix it.