One of the things I've done to straighten leather pleats it to use weighted clips like I use when drying Utilikilts. I clip it to the wayward pleat AND the pleat behind it, make sure the weighted clip is pulling on the leather, and let it hang out over time. May be like a new baseball mitt or leather holster (hope I can say holster) that takes some break in and persuasion time.

Other suggestion comes from breaking in leather, just wiggling it a lot to loosen it, being careful not to crease it. Sort of like you'd pull and wiggle the retaining strap on a new leather holster. You might try googling about breaking in new leather...lots of how to videos about it. Might work...leather is leather.

I've been very pleased with how my weighted clips "train" the pleats on my leather kilt...

Might work...