New plaster? Who did the work? How old is it? New plaster should not crack as it cures. I would call whoever did the work to repair the problem. If the cracks are not large or deep a fresh skim coat may be needed.
Do you have a picture you could share? I usually use this product for walls and it works great. https://amzn.to/2XfhxWR
With some cracks, you need to use some tape when putting on a coat of spackle.