Mine had the same issue, Rob. There is a video on the Steyr website showing how to reset it. Undo two screws on top, free the barrel, butt it up to a magazine, nip one screw and tighten the other. They bear on opposing sloping faces so they apply an axial force to the barrel as they clamp it down.

The mags are such a good fit it only needs the tiniest movement to turn a sliding fit into an interference fit. A variation of just a few microns on mag width might be the case.