Getting my babies to use the cat flap - advice please.

I have three very pampered cats who are about three years old. I recently installed a cat flap so that they can enter the house when I’m not home. I’ve had it since July and have been encouraging them to use it daily.

However so far they will only try to use it when I open the flap for them 🙄 and otherwise just use it as an expensive window.

It’s a microchip activated catflap that only allows them to enter from outside and prevents them from leaving the house after 7pm. (They have a curfew).

Any advice warmly received. (I know it’s more usual in the US to have indoor cats only but in the U.K. we’re all about the outside cats).