It’s probably not screws, cables, alignment or anything else. There is a problem with some X9 rear mechs: the spring simply isn’t strong enough to pull it to the end of its range (or the rivets are too tight, it amounts to the same thing). I have one and it had the same problem really badly when I first got it, I thought I was going mad having tried all my usual derailleur voodoo to no avail. I sort-of fixed it finally by pulling it off the bike and spending half an hour flexing it and soaking it in WD40. Then I whacked the rivets with a centrepunch a few times. It’s much better now, but in any significant amount of grit or mud the problem resurfaces.
I’ve just ordered a new, different mech to replace it because I’m a bit sick of it. If I were you I’d go back to the shop, show them the problem, and get it swapped for something that’s not SRAM X9.