Why is the front flap on my waxed canvas messenger constantly curling/rolling up?

I've got one of your waxed canvas messenger bags- the reissued version you put out a year or two ago. I've noticed that unlike my old ballistic nylon one and my even-older cordura one, the flap constantly wants to curl up around the edge.
I probably sound ridiculous, but it gets to be a pain in the ass. It's as if once my bag got wet a few times, the canvas shrunk? Since the liner on the flap of the waxed canvas bags isn't actually attached to the canvas (i.e. sealed to it the way it is on the ballistic messengers) & the liner is a heavier weight compared to the ballistic messengers, the canvas has drawn up, but the liner obviously hasn't; hence, the curled up edge.
Any thoughts?

-thanks