Sounds like the thread isn't coming off the bobbin correctly - and it is too late now , but I would never adjust the bobbin tension , that one can be very tricky to get back to where it should be
Wind a new bobbin , read the manual to make sure you have it coming off it correctly and through all the little nooks and crannies that bobbins have
rethread the machine . Also when putting in the bobbin - clean the area really well , you never know when a thread has wondered away from sight
I use Q tips to reach places I can't see