My Oceanic BCD ( Balance ) used to do the same thing when it was new. It became quite humorous after a while, I would lay my BCD on the ground, slide the tank into place, wet the strap, and two guys would tighten the strap as tight as possible. We'd wait a minute or so, and then re-tighten. I would literally have everyone in my group check (confirming it was tight) and then I'd put the BCD on, and walk down to the dive site (shore diving in Puget Sound). Invariably, my tank would slide out and hit my butt, long before my feet hit the water (much to the amusement of my buddies).
The oddest part was that my wife's BCD ( a Libra, which is really identical ) never had a single tank slippage, even though it was rigged, tightened and used identically. After becoming very adept at replacing loose tanks undewater, I finally installed a rubber tank-gripper-pad on my band, and it stays tight as a drum now. The one I use was from a Mares BCD, that my dealer ordered for me, but there are several that would work.
I'm also farely certain that the strap has streched out enough (after about 100 dives) that it would probably not slip anyway, but the Pad has saved me lots, and lots of greef!
Your milage may vary,
Boydski