Assuming there are no broken parts, the problem you're having is generally the result of either adjustment being needed to the rod that connects the outside "trigger" to the door lock, or a good cleaning being needed of the mechanism (door handle and lock). Has the level of difficulty been gradually increasing? If so, it suggests that cleaning is needed, and not adjustment. The adjustment, if it's needed, is theoretically simple but actually quite a pain, as it involves removing the interior door panel (not as bad as it might seem) and then adjusting a threaded plastic piece in the door for which access is almost nil (particularly for reassembly). I've been through all of this, so I can provide more info if you need. As a first step, though, I'd remove the door handle and then clean and lubricate that part of the mechanism. Removing the lock from the door can be difficult (the bolts have Loctite on them) but a careful job with a good spray cleaner and lubricant can do wonders.
