MacBook Pro A2159 new battery not working but old battery does

I recently replaced the battery in my MacBook Pro A2159. The new battery is not working (the laptop won't turn on), but when I reinstall the old battery, the laptop works perfectly fine.

I have already:

1. Transferred the battery board data cable and ensured it is seated correctly in the ZIF connectors.

2. Verified that the T5 power connector pancake screw is snug and making a good connection.

3. Performed an SMC reset.

4. Tried leaving it on the charger to wake it from a potential deep sleep state.

Everything is snug and tight, but the new battery still shows no signs of life. Is there anything else I should check, or is it likely a defective part?