Locks and keys are the usual suspects of most security problems even if we're talking about French doors. Installed to add a nice French style at homes and facilitate movement, French doors are often installed in porches and balconies. In other words, just like regular external doors they're exposed to climate fluctuations and may not lock properly. Before you start disassembling the security locks when problems strike, take a minute to consider whether the latch is not aligned due to other problems. It's not always the lock's fault.
In some cases, the French doors won't lock if the material of the door is warped or the door sags. Since most French doors are made of wood, the material would warp after several years and due to lack of treatment. The hinges ensure that the door is aligned and doesn't bind might get loose or too tightened. If the door doesn't close well, it won't lock well either. So, before you even consider lock change or repairs, make sure what causes the problem.
Regular French door lock repair would include the alignment and lubrication of the bolt. The doors will sag even if the door knobs are loose. In this case, you just have to tighten their screws. If they're not loose but the lock sticks, you would have to remove the door knobs. Remove the screws from each side of the door pulling out the shaft connecting the two door knobs, too. Use canned air to remove debris and dust and lubricate the security door lock mechanism.
French door latches provide extra security but since they consist of a number of parts, they need good maintenance, too. French doors often have barrel bolts acting as latches at the top and bottom part of the door, which remains stationary as the other one is used for daily traffic. You must make sure the latches are aligned with the hole at the header and floor. Sometimes, alignment problems begin with poor lock installation. If the bolt does not meet the hole at the top or bottom, you might have to remove the mortice plate and place it at the exact position for proper alignment with the bolt. You probably won't have to move it far enough. It should suffice to use a chisel to enlarge the mortice just enough to move the plate a bit farther away from its current position. When you test French door locks, avoid using force. If damage is inflicted, you will need to get new locks.
Locksmith Watford, 01923 381 024, High St, Watford, United Kingdom, WD17 2ER, http://www.locksmith-watford.uk/