As with any other door, French door locks are susceptible to jam. Over time, dirt, grease and other buildup can gather in the mechanics of the door knob and affect the lock's ability to function correctly. Trying to force the lock can break or damage the door knob of the French doors, but hiring a professional locksmith can cost you quite a bit of money. With the correct tools, however, you can fix this problem yourself without throwing down the money to spend on a locksmith.
Things You'll Need
Canned air duster
Disassemble the door knob that contains the lock on the French doors, using a screwdriver to loosen and remove the bolts so that each half falls away from each other.
Spray the inside mechanics of the French door knob thoroughly using canned air duster to dislodge any dust or other large buildup.
Douse the mechanics of the lock itself with spray lubricant. Turn the lock back and forth as you spray so that the lubricant can get deeper into the mechanics and allow the lock to slide free.
Re-assemble the French door knob by bringing the two halves back together and securing the bolts with the screwdriver.