The usual culprit is lint!
Be sure to blow out the thread cutter blades (the section where the puller arm draws the thread to cut) every day. If you do not, the lint will build up between the wave washers (you can’t even see it!) and cause the top blade to raise up just enough so that it won’t always cut. DO NOT LOOSEN or REMOVE THE BLADES!! If you blow the lint out from the back of the blades, it will shoot the lint out the front, and you will not need to loosen the gold adjusting screws. If you don’t do this, it will tremendously shorten the life of the little wave washers between the blades.
If the thread cutter will not cycle, the needle may not be in the correct position to allow it to cycle. Try using the needle up/down function to lower the needle and raise it back up one time. Then try the thread cutter again.