Your condensate drain is connected to the house sanitary drains, right?

Are they usually? I know very little about plumbing.

Here in Florida where it doesn't freeze, we just run them outside.

But up north or in big buildings, you find them plumbed into the sanitary
drains.

You may also have a venting problem on your house drains.

How can I figure out if that's the problem?

Aside from obstructions, incorrect designs are a problem. The condensate
connection may be an afterthought, and improperly vented, while the rest of
the house is OK.

It's possible to connect up flexible clear vinyl hose, at least
temporarily, to see inside the line and diagnose what's going on, whether
the trap is siphoning.