Navratilova was left flabbergasted by Stosur's shock French Open semi-final loss to Sara Errani, but is even more flummoxed by the Australian's appalling record at the All England Club.
Despite boasting one of the best serves in women's tennis, Stosur has progressed beyond the third round just once in visits to London's SW19 grass courts.
"I would like to ask her what the heck is going on with her at Wimbledon. She's five and nine at Wimbledon lifetime," Navratilova said before Saturday night's Roland Garros final that everyone bar Team Errani expected Stosur to feature in.
"It's like she should be winning there more than here, but she has more confidence on clay."
Navratilova put Stosur's surprise demise in Paris down to being "nervous playing somebody that she's never lost to before, she should beat".
"But you have to win the matches that you're supposed to win and then you have to win the matches you're not supposed to win to win a slam.
"She did that last year at the (US) Open against Serena (Williams), but couldn't find her best tennis.
"She's still streaky and I don't understand."
While Stosur has been lauded for her powerhouse game, Navratilova - who does actually believe in the Australian - insists the 28-year-old world No.6 needs to be more of a lightweight than heavyweight to realise her full potential.
"If I were her coach, I would concentrate on footwork, just becoming lighter, more of a featherweight boxer rather than a heavyweight," the 18-times grand slam champion said.
"She's powerful, but she needs to feather it. If she would have been able to do that yesterday, I think would have been a different story.
"(When) you get nervous, the feet stop moving even more so. But if you're light on your feet to begin with, even if you get nervous, you can still handle it.
"So technically she just needs clean up her game a little bit.
"But most of all, she just needs relax a little bit and not put so much pressure on herself.
"She did that great in New York; then she failed in Australia (on a) very similar surface.
"Here, her best surface, I mean, she's great getting to the semis, but she should have won yesterday.
"But there's not much wrong with her game. She's 27 years old. She's got plenty of time. She's in her prime. She's got like four, five good years left."