Just found out from the DCUO forums that the Promethium lockboxes are capped per character per month. This explains why I haven't gotten a lockbox for about 3 weeks. Time to play my hero.
Edit (18-07-2102): The cap is 15 Promethium lockboxes per character per month. The cap resets every month on the 20th. (Mepps also confirmed this on the forums, but I can't recall his post) Happy hunting!