What solutions are there for a tyre that won't stay seated? I've got a Schwalbe snow and ice 700x38 (also marked 40-622) on a Mavic A319 (19&nbsp;mm internal width).  This, according to Sheldon should be an ideal combination (and if it wasn't, the failure modes don't match).  Other tyres seat well on the rim but this one has always been problematic: when I first fitted it last winter, and the first time this winter, the bead wanted to ping out of the hook groove.  Last night I topped it up (to somewhere in between the min and max recommended pressures), got about 10&nbsp;km and it came off the rim.  After that I just couldn't get it to seat - pumping it up hard it came straight off the rim, pumping it up soft it came off within the first 10 metres.  After several attempts (over an hour in -4&deg;C) I had to give up and call for a lift.

Other tyres on the same rim are fine (marathon plus 32, marathon mondial 35).  This tyre seemed OK on the front rim for the same bike (a 17mm Alex) but doesn't fit under the mudguard (I've got a 35mm studded tyre on there).

I'd really like to refit this tyre tonight given the weather forecast, and studded tyres are only available mail order so there's no chance of getting a replacement quickly.

Is there some way to persuade it to seat and stay seated?  Does the tube width make a difference - my spare was only recommended up to 35mm but it orginally failed with one recommended up to 40mm - perhaps by exerting lateral pressure into the seat immediately rather than radial first it was persuaded to seat but only just.

Update: I've tried it on the front wheel of my 29er hardtail.  It won't seat there either, and in a few attempts it popped off in a different place each time.  The bead feels OK, less pronounced than some of my other tyres (which are mostly Schwalbe as well) but not enough that you'd notice unless you were trying to find something.
I have had this problem with two different Schwalbe tires now. On both of these tires one of the bead cables/wires seem to have become undone. Not sure how to describe it. If each of the two beads is made up of several wires wound around each other, then on both tires one of these individual wires was poking out of the bead. It was very easy to spot because on both tires it was sticking out several inches just on the inside of the bead. No apparent length wise gash. Rather, it seemed to me as if one of wires had become unwound and migrated over time, I assume, as either a wire individually, or the whole set of wires, resulting in one wire being located parallel to the other wires but outside the tire proper.  Both wires were on the inside of the tire. Sent a picture to Schwalbe and received replacement.