Sounds like someone messed with the disconnector and things aren't catching right. It's a chore to disassemble a decocker, so you might need to take it to a gunsmith or return it to the seller for your money back. If that's the case, then pretty unscrupulous of the seller to claim that it's stock.
Then again, it could be the springs under the trigger bar binding up between there trigger bar and frame, causing the bar not to rebound fully up and leaving it in the 'disconnected' mode.