Patchy finish?

I have recently painted a bedroom for a customer. The walls were magnolia and because she wanted a matt white finish I painted first with super cover paint and then two coats of matt white emulsion. I used a roller which is what I’ve always done on walls. Now it’s finished when you look at the wall from the side it looks patchy. The flat is about 18 years old and the walls are plasterboard finish. Any ideas as to why it looks how it does? Thank you for your help. Hilary