I had primed the whole room, ceiling and walls. Then after a few days, I put some painters tape up and painted the ceiling. Problem was I left the painters tape on too long and when I peeled off the blue tape, along the top of the wall (where it meets the ceiling) there are some areas where the tape pulled the primer off the wall.
I don't want the wall to look like I painted over peeled away areas, but the peeled primer spots are very small.
Can I simply reapply primer on those spots? Or do I need to sand down where the primer is peeled, and then reapply primer?
I would apply the primer to the peeled areas, wait for it to dry, and then assess if it is noticeable. Chances are it will not be, but if you want to touch it up, fill the peels with drywall mud, sand them when dry and spot prime them. You may have left the tape on too long, or you may have applied it too soon after the prime coat.
As a side note, there is really no need to tape the walls when painting a ceiling. In fact you want the ceiling color to come on to the wall a little as it makes it easier to cut a straight line between the walls and ceilings, and ensures there will be no gap between the two colors, which is often very noticeable. It also allows you to paint the ceiling faster.
Was the wall painted already or is this fresh drywall??
In both cases I would say you need to do a better job preping the wall and making sure it is clean. Primer will not stick to some areas if it is not clean and dust free. Painting is 80% prep and 15% painting and 5% What paint you use.