Why does my car start once after disconnecting battery

I’ve just replaced the battery in my 2006 vw GTI. Car starts once after having battery disconnected for a bit, resetting ecu I’m guessing. Runs fine when started, when I turn it off and go to start it again, it churns, turns on and dies. Try again, and it won’t start and churns horrible and progressively gets worse until I disconnect battery for a few hours