When you insert your key into the lock and it is difficult to turn, or even to slide the key in or out of the keyhole, the lock probably needs to be lubricated. To confirm this, try using the key when the door is standing open. If it is still difficult to operate, then the problem lies in the lock.
There are a couple different types of lubricants that can be used in a lock, but we prefer graphite powder. Graphite powder is odorless and tends to stay in the lock rather than sticking to the key. If you use a petroleum based oil, it can make your keys oily and worse, dust tends to stick to it inside the lock and become gunky. However, whichever you choose, stick with that lubricant. Mixing lubricants is a sure way to gunk up a lock.
Graphite powder comes in a small squeeze tube. Simply uncap the tube, place the tip of the tube in the keyhole, tip the tube up slightly and give a couple of squeezes to blow the powder into the lock. Now insert your key repeatedly and turn the lock repeatedly to work the powder into the lock. You should also lubricate the bolt by squeezing graphite into the cracks around the bolt or latch on the edge of the door.
The lock should operate more smoothly now. If it is still difficult to operate, the problem may lie in the doorknob itself or the latch mechanism which will require disassembly and inspection of those components.
