I would agree with CorpseQueen on checking the fat. Butter tends to make a denser cake unless you whip the heck out of it, but of course it tastes better than shortening. (Can’t speak to using oil in a yellow cake, haven’t done it.) However, I’d definitely look at your acid/base ratios. It’s possible that you need to adjust by adding a bit of baking soda if you’ve got acids in there such as sour cream, lemon juice, etc.
You can also spend a little more time beating in your eggs; a couple of minutes each on a stand mixer adds a ton of air and helps incorporate the ingredients. Finally, you can try splitting when you add the eggs: beat yolks into butter/sugar and then fold in the whipped egg whites.
Good luck!
