Elna SU missing stiches and breaking thread

Question

My elna SU ( about 1979 ) got the thread tangled around the outside of the bobbin. I loosened two screws and then was able to get the thread out. I then put the two screws back in and then the bobbin and then rethreaded the machine. If I sew a straight line, it misses picking up the bottom thread. If I try to do a zigzag stich, it breaks the thread. Please help!!!