SUPER-important point. Certain areas that respawn will respawn in their entirety. Meaning all items in the area will also respawn. This has been a stable Bethesda mechanic since Oblivion. If you're having trouble getting the higher tiers of power armor, return to one of the INTERIOR areas where you found a suit. If there are new baddies waiting for you...there's also a new suit at your level waiting in the same spot.