Roller paint marks crop up for a variety of reasons. Often, the culprit is cheap paint. Low-quality paint has a tendency to be thick and does not spread as well as higher-quality paint. Consequently, you often find lines of thickened paint on the wall. Other times, a cheap paint roller or user error is at fault. Whatever the culprit, the best way to get rid of roller marks on your wall is to paint the surface again, this time the right way.
Sand the walls with a fine sanding sponge (120-grit or so should be sufficient). Sand enough to roughen the surface of the paint on most of the wall. If your paint roller marks are raised (a sign of paint that is too thick), sand them down to grade level.
Wipe the walls down with a tack cloth to remove any sanding debris.
Tape the surface of the wool nap roller cover with masking tape. Then peel the tape off. This will remove the loose fibres on the surface of the cover before they can end up on your wall.
Pour the paint into the paint tray and load the roller. Simply wet the nap in the paint reservoir then roll the cover over the tray to apply the paint to the surface of the nap. Wet and roll the roller again if necessary to apply more paint. Never submerge the roller itself. Paint can seep inside the roller cover and get squeezed out onto the wall, creating roller marks.
Apply the paint in broad strokes using light pressure. Never apply more pressure to squeeze the paint out. Instead, keep the roller loaded by dipping it in the paint tray frequently. Roll on one broad stroke, then back roll over the stroke to add a second layer (never overwork the paint by repeatedly going over an area). Then move on to the next area of paint, overlapping the first by an inch or so. Work in adjacent sections all the way across the wall to avoid covering dry paint with wet paint, which leads to roller marks.
Allow the paint to dry for the manufacturer-recommended amount of time before adding a second layer (if desired).
Paint conditioner can be mixed into low-quality paint (according to the manufacturer's instructions) to thin it out and make it easier to apply.