It is quite easy to dislodge one of the pads out of its retaining groove in the caliper bracket (particularly the non-piston side) when refitting the wheel, especially if you have used a lever to get the pads apart. Check both are located properly in their respective slots.

Another thing that's happened to me (not on the XT mind) is that, when sliding the disc between the pads, one of the friction surfaces has broken away from its backing and fallen off, where the bonding has failed! Have you still got both friction surfaces still?

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