Plastic threads can become broken or worn over time or if screws or bolts with the wrong sized thread have been inserted into the threaded hole. The threads can also break if the screw/bolt is not inserted at a 90-degree angle. The problem is solved by using a tap to rethread the hole. Taps have different sized threads for various sized holes and can be bought in kits with an array of different diameters.
Insert the correct-sized tap into the beginning of the threaded hole. Slowly turn the tap clockwise one quarter turn.
Turn the tap counter-clockwise to remove any plastic shavings.
Turn the tap another clockwise quarter turn, before turning it counter-clockwise as before. Repeat this process until the plastic thread has been fully rethreaded. Remove the tap and slowly install the screw/bolt into the newly-threaded hole.
Make sure that the tap is inserted into the thread hole at 90-degrees to the hole and that the correct diameter tap thread is used. Note that the tap thread is tapered at the end to allow easier access into the hole.