Andy has the issue exactly right, it's called ghosting or witness lines which occurs with poly.
Once you are happy that the paint is flat, do a last thin coat, maybe 2 if needed, then buff that. Typically with cellulose (nitro) you would spray many thin coats, 12-15 is what I normally do, but with poly you tend to spray just a few much thicker coats so when you come to level sand you never go through the coat you've just sprayed. The trouble with wipe on poly is that it's very hard to put on a thick coat with a rag or brush and have it go on level enough to not require a load of level sanding, so it gets applied in lots of thin coats like you would with cellulose but without the benefit of having the coats melt into one homogeneous layer.
FYI you can thin and brush on cellulose and have the benefit of the coats melting into each other.