You should not have to repair cracks in new plaster, these cannot be repaired by yourself and I would get the plasterer involved as if he has pride in his work he will sort it out! If I had to guess I would say a strong mix of say 3-1 which although most people say to do it it doesn't work well. Im really curious on how he prepe'd the walls 1st as I know what I would do but I don't know what he did?