Cat won't go thru cat flap

Well, my cat flap is just a hole in the wall, actually...

My kitty cat gained weight last winter. She wouldn't go outside thru the hole any longer (once she was stuck halfway). 

She's now lost some weight, but she's kind of traumatized : although I made the hole somewhat bigger, she just won't try. Pretty annoying, since she should be able to go outside and get back in, when I'm not home.

What should I please do to make her understand that the hole is wide enough for her ?

How to trick her into attempting again to go thru this hole, please ?

Thanks a lot !

Are you sure she fits through now? 

Best use patience, and treats. You have to grab her and shove her through the hole. Have a treat waiting on the other side. Do that once or twice a day, no more frequent so as not to annoy her   Repeat for at least a week. Longer if necessary. 

don't grab her and force her - it'll make her trust the hole less.  after making SURE it's big enough, ust put a can of tuna on the other side of the hole and close the door so the only way to get to the tuna is to go through the hole.  chances are after a while she'll try the hole.  .