Why do touch ups on flat paint show

When you say you can see the brush marks, do you mean that they're actually irregularly surfaced? I mean - are we talking just a visual effect or an actual difference in the depth of the paint?

If the latter, do a skim coat with lightweight joint compound and when it dries, either lightly sand OR smooth with a large, slightly damp sponge, then apply primer.

As for your own brush strokes - orient the roller so the roller bar leads your motion. If you're moving right, put the bar on the right. If you're moving left, put the bar on the left. The most pressure will be on the roller bar side, so keeping that side on the "front" of your motion will mean the lighter touch side (the back) will smooth out the uneven strokes as you pass over it again.

Now if its not a real issue with texture, just that your seams are obvious - then just prime it again with a good HEAVY primer, using the proper brush techniques, and you'll be fine. :)

Only a guess but maybe you had some of the adhesive still on the wall and painted over it causing the sticky feel. If so, you may need to sand it very carefully just over the touched up spot and surrounding 1/4 inch to ensure the area is smooth. Dust off with a dry soft cloth or dusting brush and try touching it up again just over the sanded area. I would use 180grit or 240 grit where oil based paint is concerned. You may even try wet and dry paper with a little water to lubricate the sanding.

Touch ups are hard to get right but don't worry, you do need to make it ugly before it becomes beautiful and sometimes when you try to touch up a postage stamp sized area and the whole wall is only 30 square feet, believe it or not, it can be easier to just bite the bullet and paint the whole wall.