Why can I see roller marks after I painted?
Most likely, you tried to cover too much area between dunks of the roller in the paint.  The later strokes have less paint, so you're seeing some of the underlying color come through, whether it's primer or another layer of paint.

Some tips that might help:


go back to the roller tray more often.  FWIW, I generally do four to five strokes with the roller between dunks, and do three overlapping sections over the height of the wall, so maybe 10-15 sq.ft. per dunk.
I always count on doing a second coat.  I prefer a couple of thin coats to one thick coat; after the first one has dried, it gives you a chance to see where you need to pay special attention on the next coat, and (for me anyway) it's difficult to get an even coat.
with a freshly-dunked roller, I will start a couple of feet over from where I stopped the previous time, roll back towards that point until I overlap, then change direction and roll until I've gone a couple of feet past where I started.