Uh-oh. According to a support doc released yesterday, T-Mobile has paused the Optimus L9's update to Android 4.1.2 Jelly Bean because of the new software's less-than-stellar stability. Several users have reported significant battery drain, difficulty receiving calls, and frequent app crashes. Accordingly, T-Mobile wants to "improve performance" before resuming the OTA upgrade.
T-Mobile announced and began rolling out the Optimus L9 Jelly Bean upgrade on April 22nd, so it's somewhat surprising it took so long to identify problems with the update.