Diagnosis
The most common cause is a worn or failing battery. The brushless motor and electronics are likely fine.
How to Fix It
Test with a different, known-good M12 HIGH OUTPUT battery. The 2505-20 needs the high output pack for full performance. If power returns, your old battery is done. If still weak, clean the tool's battery terminals as described earlier.
If That Didn't Work
The motor's Hall effect sensors may be dirty or failing. This causes poor motor timing and power loss. This is a service center repair.
Heads Up
Do not confuse this with the clutch slipping. Make sure you are not in a numbered clutch setting. Set the collar to the drill icon for maximum power.