It also happens that cats lose their chip. That's what happened to us. Maybe check with the vet to see if it's still there...

I have the same problem with my two cats. I've changed the batteries several times with new ones but after a few hours the light flashes and they can't get in. As it worked the 1st week, I threw the box away...

Re-check the batteries. I don't know where you live, but if it's very cold, the batteries sometimes only last a short time. I changed my cat flap after more than 10 years, because I was also having problems with the batteries wearing out quickly in winter. With the new one, it seems to work.

If it doesn't switch even when you hold it down and you haven't switched it off by mistake, I assume the thing is broken and needs to be repaired or replaced...