What does "She flat-tired on some junior varsity b*s*" mean?

I've watched Grace and Frankie, SE1 EP7. I liked that but I don't understand this sentence.

She flat-tired on some junior varsity bullshit.

Frankie answered her phone while she'd been watching 'The Spelling Bee'.
Here's the part of the script for context.

Frankie: Hi, Mike.

Mike: Is this a good time?

Frankie: It's always a good time to talk to you.

Mike: So how is everything going?

Frankie: Well, you just missed Marinda just before. She flat-tired on some junior varsity bullshit. Her parents are going to kill her!

Mike:- And now... I'm sorry? What?

Frankie: Well, you know, the spelling bee!

I think that means Marinda did something wrong. I would really appreciate it if you explain it.

It means "she failed at an easy task".

"To flat-tire on something" probably means to fail, evoking an image of a race or progress coming to a halt.

"Junior varsity" is a secondary team of younger and less experienced players at a competition.

It was an easy question that even a junior player should have gotten right but she got it wrong.