Why is it so hard to lock the door with the key?

It is nearly always because the door has gone out of alignment and the bolts no longer go straight into the strike plate on the frame. They rub, and you have to force the key to throw them.

The most common causes:

• The door has dropped a little under its own weight, and the hinges need adjusting.
• The frame has moved with damp or with changes in temperature.
• The strike plate is badly positioned or has come loose.

A clue: if the key turns smoothly when you lock with the door pushed, the problem is the adjustment, not the lock.

It is best not to get used to forcing it: that wears out the lock and the key can snap. With an adjustment to the door or the strike plate it usually ends up as good as new. We look at it in lock repair.