Yeah..... seriously - I've never dropped a bottle yet despite banging and crashing down some decent Galloway hills, and round dalbeattie stanes etc, so far i've only used the 500ml bottles on my mtb as they generally last a few hours or i can always top them up from a decent stream or hill spring, i've never tried the larger 750ml bottles off road yet but they've worked fine on the road bike without any movement or suicidal leaps from the mount.
Mine snap into place wi reassuring "thunk" and need a tug to free them out of the mount so perhaps the moulding around the metal disc in your bottle was a bit dodgy, i know they have changed the bottle design from the original red capped version.
EDIT : I've just checked my bottles as i have 4 of the original 500ml red capped bottles and 2 new 500ml + 2 new 750ml versions with the black capped improved valve system (much improved) and the metal disc insert on the bottles is far more pronounced on the new versions so i guess it makes better contact wi the frame mount/magnet? - give the new bottles a try before you write it off
Perhaps i've been lucky or maybe i ride like a big mincing wuss but i've had nae probs wi them at all.