Cat Door Training

My wife and I adopted two stray/semi-feral kittens about a month or so ago. As my wife is terribly allergic to cats, they are going to be outdoor cats. We have had them in a rather large shed in the backyard for the entire time we have had them, and per advice I was given in the stray forum, we confined them for the first month and didn't let them out except to see the vet. This weekend, I installed a cat door - real simple, locked or unlocked only - and now the darn things don't want to or can't understand how to use it. I can push them through it, but then won't use it to come back in. I've tried leaving one in when the other is out, but then they just whine, and the outside one either waits by the front door to go in, or runs around.
Does anyone have any ideas on what it takes to get the cats to use the cat door?

Can you tape the flap open for a bit so they can see what's on the other side?

I put a cat flap in the kitchen window screen this summer and it was a while before all the cats used it. One or two were quick to adapt and some of the others took a little longer. I did push a couple through the flap both ways so they would get the idea, but I know cats like to eye things up a bit before they take for granted that it's a good thing for them.
I also held the flap open for them to come through each way a few times so they could get the idea to use that instead of needing me to play doorman

Thanks. I think you are right about them liking to see through. When we prop it open with a stick they seem to be ok with going back in. I also partly think they know my wife is a sap, and will continually open the front door of the shed if they play sad/stupid with her.