I don't usually post on here, hope its ok
I have two kittens, about 5-6 months old and have recently started going out on their own.
My problem is one of the cats uses the catflap fine, comes in and out when she pleases but the other one will go out, but won't come back through the flap and cries until we go fetch him or will just stubbornly stay in the garden until we go looking for him.
Any advice would be great I tried google but it wasn't very helpful.

What I used to do with mine was sit on the inside of the catflap and open it inwards, like it would be from the outside, hold it open and encourage my cats back in. Use food or a favourite treat, even a mouse on a string toy if necessary but eventually your cat will get the idea.

My back door goes down a step so the flap is higher from outside. I have a couple of bricks there so my cats can step up better to reach it.

Good advice ^ remember to gradually lower the flap as they get used to going through it so they get used to pushing a bit and feeling it on their backs until the reluctant one will do it himself.

Maybe the reluctant cat is also less motivated to come in than to go outside to have fun?

I think that flakecake and Tass have offered sound advice, but bear in mind that cats like to try it on, while reminding us that we are their slaves.

So don't stand for excessive cheekiness. She will come in when she's hungry. In fact, don't feed them before they go out by way of encouragement to use the flap.