Our Sureflap stopped working last night. Cue a very upset cat who couldn't get in for breakfast. We changed batteries. We reprogrammed the cat. Twice. At which point he made it Very Clear he'd had enough of that, thank you.
I sat and whistled like a loon for about half an hour to get him to come in with no luck. I bribed him. He ate the bribes and stayed outside.
Then upon reading the faultfinding page in the manual, it said to make sure there isn't any interference from anything.
The penny dropped. Last night I installed a smart plug about 40cm away to control some outside fairy lights. Took the plug away, and now the catflap is back to normal.