Seat Tilt?

Hi all. Hope this is an ok place to ask.

I have a brand new schwinn meridian. Assembled last week, ridden less than one mile.

Everything else I have figured out so far. But no matter how much I tighten the seat bracket, the seat tilts back when riding. I understand sometimes the seat bracket gets stripped, but this is brand new, so that shouldn't be the issue.

I am a pretty big person but the tribe's weight limit is significantly higher than my weight, so I don't think that is the problem either. Plus, the seat doesn't sink down, it just tilts.