I've noticed that in Borderlands 2 there are many DLC missions whose level gets set the first time you visit their location. While I was just wondering around, I visited them all, when I was low level, so they are all now stuck at level 15. Can I just reset them somehow?
It doesn't seem like you can without resetting ALL your quests, and even then not on your first (normal mode) play-through.
On the main menu screen if you choose "Select Character" you should get a menu that looks like this:
There is an option to reset on this screen (the "Q" key on PC, "A" on xbox and Triangle on PlayStation)
I noticed that since you are specifically mention level 15, you most likely have not reached True Vault Hunter Mode (TVHM) or Ultimate Vault Hunter Mode(UVHM) (each mode is unlocked by completing the previous play-through). I know it is available for UVHM and not for normal mode (I didn't have a TVHM character to test sadly. Perhaps someone else can try and edit).
Your next play-through will have all the quests at a higher level available for you and I believe that UVHM will actually have them at whatever your current level is. So if you reset at max level, you will have the missions at max level.
The reset (as I recall) applies to DLC missions as well.