Hi! Madderbat,
It Could be worth checking the temperatures of the two brews, it might be that the ones in the window warm up above airing cupboard temperature during the day if they are in the sun. Or the airing cupboard ones could be too warm for the yeast to work at its best. The airing cupboard temp could also rise and fall with hot water demand. The quickest brew should be the one with the most advantagous stable temperature.

What yeast are you using or re-using ? I will check on the temperature range that it requires for optimum performance.