Crazing on new render?

I have just had the house re-rendered, Initially they were going to remove the old render and apply new, However, some areas where still sound, therefore the builders just applied a scratch coat and then a smooth finish coat. I now have several areas where the plaster is crazed and when you tap it sounds 'blown' to me. I am being charged gbp 9000 for the rendering (detached sandstone house), so was expecting a good job. I am being told that this is normal, but it is really hard to prove. Can anyone offer advice?? Desperate.