X,Y scale with Z: print starts off right size, then narrows

This is a Prusa i3 printing PLA at 190C, bed temp. 70C. The printer was working without problems yesterday, but today I moved it to a new location. Nothing was changed in the software or wiring.

The symptoms are that the first layer prints fine, then as Z increases, the print head appears to be travelling too short a distance, so the width of this 1cm tower reduces, and in the end the nozzle gets in a mess.

Any ideas what the problem here is? It looks mechanical to me, but I can't see any binding or skipping on any of the axes.

Thanks!

The problem turned out to be the PLA filament - it must have absorbed moisture. Changed to a different reel, and everything now looks good.