I've noticed (along with many other hunters) that my pet wasn't holding aggro as well as he did pre-2.4. I respecced my wind serpent (just to make sure that that wasn't the issue) with Growl and Lightning Breath, and turned auto-cast off for LB. I made a combat log filter that showed only actions done by my pet, and sent the pet in to combat with several mobs. Growl was never displayed in the combat log, although my pet's focus was going down by 15. Lightning Breath, on the other hand, does show up in the combat log.

If you have "everything" turned on in the combat log will you see Growl?

It appears Growl is considered a spell unlike other pet abilities and is not turned on by default in the "Self" filter. This doesn't seem to be intended and we're looking into whether it needs to be changed.

You can turn on Growl in the combat log by selecting Settings under Filters in the combat log tab and then selecting the Message Types tab and checking Spell Casting.