Some other possibilities:

Gooey dirt or scum on the rod guides.

Dirty line. I see this a lot with customers who claim their line doesn't shoot. It's usually the reason my lines don't shoot as well over time.

Worn line.

Low quality line. Better quality lines have better better coatings and shoot better. Even the least expensive lines from Scientific Anglers, Cortland, Rio, Airflo, Orvis usually shoot well when new out of the box. If they get dirty they do not shoot as well. Some of the inexpensive lines packaged with combos do not shoot very well at all.

Limp/soft line. Some cold water lines get limp or soft in warm/hot conditions and do not shoot through the guides well.

Don't have enough line out side the rod tip. Another common issue. A weight forward line will shoot much better once you get the entire belly and some of the back taper out of the rod tip. Depending on the specific line, you will need somewhere around 32-36 feet of line outside the tip to get into the back taper. More line helps load the rod better too. When it unloads it generates the line speed necessary to shoot line. Most of the line manufactures have specifications on their website where you can see the details of the line taper.

The extra mass of a heavier line may help slightly with shooting line in some situations but won't correct for casting deficiencies. In my experience the dealing with the other causes I listed will have a greater impact on shooting the line than going up a size.