You have a broken link in your signal chain. something is no longer functioning as it should. You've already verified that the mic is OK, by trying it out in another system (your amp). You have to verify each link in the chain, one at a time, until you find the piece that doesn't work.

Try feeding a 'known good' signal source into the mic input of your sound card. You verified one mic, but you have two of them, right? Do both of them work with your amp, but not with your sound card?

There aren't many links left to check out. 'Looks like it comes down to either your adaptor or your soundcard. My guess is that the jack on your card is not making good contact with the adaptor.