How to repair the sewing machine not hooking the bottom thread

Look at the lower thread spool to see if there is any broken thread stuck in it. The lower thread should not be wrapped too tightly. You can change the spool to test, open the back cover of the sewing machine, and see if the bearing screw on the crankshaft is loose. Because the needle is sometimes stuck on the orifice plate above and below the needle, burrs appear on the wall of the needle hole, which clamps the thread. Just remove the board and clean it with a sandpaper and a file. The pinholes can be used continuously even if the pinholes are not round.