are you trying to do this on the same ceiling? Was the ceiling just painted? What paint/sheen was used?

It's the same ceiling - different location. The ceilings have been patched and painted over in the past but not by me. There are a couple spots that I've just seen and wanted to touch them up. When the painter did it, it was right after he painted it - like the next day or so. The paint used is BM Ceiling Paint and it is flat/white.

You probably have to prime the spots that you want to touch up before you paint. Touching up paint requires that you use the same applicator that was used when originally painted. My biggest question is are you seeing a color difference or a sheen difference?

I guess it would be sheen - if I step back and the light hits the ceiling right, it looks dull where I've gone over it with the roller. Maybe I haven't used the right roller or I didn't load it with enough paint? I tried to feather it out.

Are you sure that you are using the same ceiling paint? Ben Moore has Muresco Ceiling Paint and usually has a contractor ceiling paint. Muresco is a softer white compared to the contractor product. It may also come in different tones of white.

This is the paint - I purchased at the BM store:

http://www.myperfectcolor.com/Benjamin-Moore-258-Gal-Muresco-Ceiling-Paint-White-p/mpc2580001.htm