Brad and Julie Vaughn wrote:

I have a 2 year old house. I have 2 upstairs bathrooms that are
having flush problems. If you flush either toilet once, you're OK.
Twice and you will have the water swirl up to the top and drain across
about an hour. Also, if you flush one of them and then use either of
two sinks, bubbles of air will come up through one of the toilets.

Yes, snake it out. Could be something like a wash rag jammed in
there.
If the snake has a spring hook end, it may be able to snag whatever.

This is *not* a venting issue. A completely blocked vent will not
prevent water from flowing downhill.

Jim