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Is the unstable one a plant you've repotted. If not try repotting, if so take it out and try adjusting it in the pot, if there are lots of arial roots try getting some (possibly not all though) into the pot to help stabilise it.
I've found sometimes just taking it out and trying again fixes it even if you don't think you've done anything different. If that doesn't work then try using a stake in the pot, you can cut it off low so it supports the leaves rather than a spike.
It is good to have them stable, they are happier that way, though sometimes I've had unstable ones which I've not needed to move (in the greenhouse I can water in place) and without being disturbed all the time they've got new roots going which have secured them so that when I've come to move them in for the winter I've found them nice and secure.