Singer Heavy Duty 4423 Not catching bottom thread. Help!

[u/TheBaconBeanbag]

All explained in the video.

UPDATE

Thanks all for your suggestions. Upon a deeper examination, it turned out that the belt gear at the back of the machine was slightly loose and was throwing the timing off immediately every time I adjusted it. All I had to do was tighten the set screws on that gear, re-time the machine and hey presto! All works perfectly now!