Unable to earn any credits in StarCraft II campaign

I am trying to play through the Wings of Liberty campaign and having fun, then randomly I start completing missions and noticed my credits was stuck at 25,000. No matter what missions I finish I never seem to get more credits. Is there something I'm missing, or is this a bug? The missions each say they have some reward like "120,000", which I assume is supposed to be earned credits (?). This started happening around mission 10-15. Playing on Normal, but played a few missions on Easy.

Solution 1:

Credits for a mission are only awarded the first time you complete the mission. There is no way to farm additional credits during a campaign, so you'll never be able to purchase everything.

The mission still shows the original reward when you replay it, but you won't get it a second time if you start the mission from the archive.

This includes when you replay it at higher difficulty, since the credit reward doesn't change between difficulties.