I have a server my buddy and I play on where we use the Mindcrack pack and we have been playing for a few days now and neither of us have gotten any Lapis yet.. Was what layers it spawns on changed or something? Or maybe we are just having bad luck.
Alternatively head off into the Twilight Forest and find a Hollow Hill, find a big one and you'll have so much lapis you'll be able to make a house out of lapis blocks.