In reply to Ben Robson: Ah, the joys of Dynafiddle! Did you make sure there was no ice built up under the toe area? If the toe piece seemed to engage but then disconnected under minimal stress it may have been ice in the toe lugs. Taking an old nail, or paper clip to clear them out is useful as a pole tip or axe are usually too big.
I've had this problem in the alps when boot packing up in the sun, and then sitting in the shade where water in the toe lugs froze solid.