Here are a few steps we can take:

1. Check for any obstructions: First, make sure there is nothing blocking the door's path, like a doormat or debris on the floor. If there is something in the way, remove it and try closing the door again.

2. Inspect the latch: Next, check the latch to make sure it's properly aligned with the strike plate on the door frame. If the latch is misaligned, it won't be able to engage with the strike plate and the door won't lock. You may need to adjust the strike plate or the latch itself to ensure they line up properly.

3. Check the hinges: If the door won't close all the way, it may be due to an issue with the hinges. Check to see if any of the screws holding the hinges in place are loose or if the hinges are damaged. If you notice any issues, tighten or replace the screws as necessary.

4. Lubricate the lock: If the lock seems to be sticking or difficult to turn, it may need some lubrication. Apply a small amount of graphite powder or a silicone-based lubricant to the keyhole and try locking the door again.

If none of these steps resolve the issue, it may be a more significant problem and we may need to call a professional locksmith or handyman to take a closer look.