1. New drywall (Sheetrock is a brand name) needs to be primed with a primer like Firstcoat or one of the other Sheetrock primers. Most builders skip this step because of the cost. I have painted for years and we only used it for about a year or two because most people would not pay the cost. Then they would complain about the way the walls look.
2.  Another thing that can make the walls look bad is a poor drywall finisher. It has not been finished enough to sand but they do it anyway.
3. A person sanding the walls does it to fast and hard and roughs up the drywall. This is very hard to fix. The walls can be sanded again after they have been primed but they may still be rough to some degree.
4. The amount of light the wall has shine on it. We can not control this! So the walls need to be as close to perfect before they are primed and painted.
5. If your walls have a knockdown finish it can be hard to match the finish if need be. I have patched ceilings as big as 3' square with fairly good results but small places on walls may show up.
6. A painter has used a roller cover that was too large to paint drywall with. A half 1/2" cover works well for most flat and eggshell finished walls. But the higher gloss finishes need a 1/4 roller cover.
   
7. A bad spray job. Splatters, blobs and runs of paint here and there. The painter should have corrected these while the paint was still wet. The painter may have over looked these but a good painter will fix there mistakesmost of the time. But things can get passed anyone at any given time.
   
8. The sheen of the paint.
9. The quality of the paint.
   

Most of the bad looking walls I have see have been from a bad finisher or sander. Some from a bad painter. If your walls do not look near perfect before they are primed or painted they will never look that way. If you paint and touch up dents and holes this will show up no matter how many coats you put on the walls. Trust me I have done it many times. I have painted hallways three and four times to get it to look halfway good. Sure they look a little better each time but perfect. Well we all should know by now know one is perfect and not paint job or anything else is perfect. We are not perfect but we try to be as close as humanly possible.

1. New drywall (Sheetrock is a brand name) needs to be primed with a primer like Firstcoat or one of the other Sheetrock primers. Most builders skip this step because of the cost. I have painted for years and we only used it for about a year or two because most people would not pay the cost. Then they would complain about the way the walls look.
2.  Another thing that can make the walls look bad is a poor drywall finisher. It has not been finished enough to sand but they do it anyway.
3. A person sanding the walls does it to fast and hard and roughs up the drywall. This is very hard to fix. The walls can be sanded again after they have been primed but they may still be rough to some degree.
4. The amount of light the wall has shine on it. We can not control this! So the walls need to be as close to perfect before they are primed and painted.
5. If your walls have a knockdown finish it can be hard to match the finish if need be. I have patched ceilings as big as 3' square with fairly good results but small places on walls may show up.
6. A painter has used a roller cover that was too large to paint drywall with. A half 1/2" cover works well for most flat and eggshell finished walls. But the higher gloss finishes need a 1/4 roller cover.
   
7. A bad spray job. Splatters, blobs and runs of paint here and there. The painter should have corrected these while the paint was still wet. The painter may have over looked these but a good painter will fix there mistakesmost of the time. But things can get passed anyone at any given time.
   
8. The sheen of the paint.
9. The quality of the paint.
   

Most of the bad looking walls I have see have been from a bad finisher or sander. Some from a bad painter. If your walls do not look near perfect before they are primed or painted they will never look that way. If you paint and touch up dents and holes this will show up no matter how many coats you put on the walls. Trust me I have done it many times. I have painted hallways three and four times to get it to look halfway good. Sure they look a little better each time but perfect. Well we all should know by now know one is perfect and not paint job or anything else is perfect. We are not perfect but we try to be as close as humanly possible.