Confused!! I am raising the hem on a pair if jeans and the bobin thread is not catching the top

Question

I am raising the hem on a pair if jeans and the bobin thread is not catching the top thread or when it does. the Top thread slips out of the hole. What am I doing wrong?? The machine make a weird grrrrr sound as if it is jammed and undo the latching where the bobin is and there is lots of thread twisted together. I tried re threading the bobin too!!