It might be because you're debating someone you know isn't really interested in a source argument. Just ignore those people or mess with them using agree and amplify tactics and other things to piss them off. That's more fun and will re-engage your mind into the fight.

It's also important to remember that in most debate you're really fighting for the audience not the opponent. Think about all the people that might quietly be reading and agreeing. That's a motivator. If you give up and don't engage you let the other asshole win over the normie readers. Don't do that. Fight for every mind you can.