Why have my water grains slowed so much?

Can my water kefir grains be tired? They look and smell the same as always, but take twice as long to produce water kefir: a week now. The temperature has not changed - it is between 22 and 28 degrees Celsius ((75-82F).
The outside temperature has risen greatly but they are kept in the airing cupboard and always have a thermometer by their side. What’s going on? Tweet