Cat won't use new catflap - help!

[PrayForMyBum]

Morning all. We have two lovely four-year-old cats who have had no issues with their catflap since they were kittens. But recently, a local cat has been terrorising them and coming into our house through the catflap and stealing their food/generally being a massive PITA.
So we replaced the catflap with a new one that uses their microchips to admit entry. This has resolved the issue with the other cat - but one of our cats just refuses to use it.
We're at our wits' end as it's been nearly two months now. We've sprayed it with catnip. We've propped it open to get him used to it. We've left treats on the other side. But nothing seems to work. His brother has no issues at all.
He's a nervous cat anyway, and it seems that the 'click' noise the catflap makes when it registers his microchip and releases the door is putting him off, among other things.....!

Any ideas what we can do?

You’ll just have to be a good cat slave and open the door at command. 😂😂

I mean, yes, obviously 😂
But while it's not really a problem day to day, it is an issue if we go away for a weekend and he gets stuck either in or out..... We've got a weekend away coming up and we're a bit worried about how he'll cope. For our summer holiday we had a housesitter in looking after them but, if anything, the situation is worse than it was before (he at least attempted to leave via the catflap previously - now he won't).

So we need some kind of solution!

My cat sometimes sits outside the cat flap and meows for me to open the door

Cat’s don’t have owners, they have staff

DCat1 took ages to get used to the microchip flap and still prefers to have the door opened. DCat2 isn't bothered and just follows DCat1.

I think it's cold turkey time OP. Make sure he's in when you go out, then if he really wants to leave he'll use the flap and you'll know he can get back in.

We had the same trouble with a neighbouring cat but that was getting in via a microchip cat flap!

Problem is now solved but DC still prefers to use the bedroom window at night. Daft thing came in yesterday from the garden (next to the cat flap) up the fence, over the garage roof, up the slippey conservatory roof in through the bedroom window then down the stairs to get his dinner

I had the same issue with one of mine. I ended up lifting him and putting him out the cat flap, it rained, he cried at the door, so my 6yo went out, picked him up and put him back through the cat flap into the house. Cat soon got the message that not mum, dad nor our elders were playing games! He was very soon over his fear once he realised there was no threat. We have 2 other cats we thought would lead the way - nope. Didn't encourage him!