I have a different problem. I use a dremel on nails. I use treats after each nail - progressing to every 2nd nail - onto once per foot - and then once when we are all done - all of this over a longish period of time.
This has worked fine with my starving dobes who apparently never get fed. I feed raw. People tell me it must be hard to give treats to a dog that gets raw meat and goodies every day. Well not with dobes.
But with the kelpie, Jill, it has worked against me. She is highly toy motivated - food, not so much. This approach has caused her to associate treats (chicken cooked with garlic, sliced string cheese, cooked liver, yummy chummies, etc.) with things she doesn't like. So now when I whip out the treat bag, down goes the tail and she tries to hide. It isn't like I do nails every day and I have never quicked her, but last time one of her dewclaws may have gotten a tad hot.
I did her nails tonight with no treats and it was no worse than the last time I did them just over 2 weeks ago (when I got a yip out of her for her dewclaw). I gave the whole thing a rest and only give her a treat when she will take one then walk away. It is taking an awful long time. Giving her food when she is excited is just plain out demotivating. But gradually, ever so gradually, she is happy to take a treat provided I have a toy on me. So it's tug, tug, treat, tug, tug etc.
What Adara said should work - to get the flap out of the picture and work on puppy going thru the opening. This is how we teach the chute in agility - get them to go thru an open hole, lengthen the fabric tunnel and then start letting the fabric touch their back until after about 3 sessions, the dog is going thru the chute and nosing the material out of their way - at a good clip. But do this slower than 3 sessions - make sure that each step puppy is very happy to go thru. before you change the picture at all.
I have a sheltie that comes to visit who has never learned the dog door. She has been coming here for 8 years and just can't manage it. Jill (kelpie) taught herself the dog door - she watched the others go thru and then at 8 1/2 wks old, climbed thru an opening that was higher than she was long. I need little steps for her even now and she is 6 mths.