There could be a lot of reasons why that happens.
First of all rethread your top thread. Take the bobbin out and put back in. Pull the thread out of your bobbin and start all over. Even if you know that you have done it correctly. Try another bobbin. If you have different ones, try another brand I once bought some cheapies and they work, but they are looser than the original ones and therefore I have more problems. Try diffent thread. Don't know if you have to oil your machine. That depends on the machine, so check your manual and see. Also make sure that the bobbin holder is sitting correctly. Take it out and clean underneath. I cannot remember ever needing to adjust my bobbin tension on my mechanical machine and it is getting pretty old. I do have to say that I don't put thick ribbon or thread in the bobbin, because then you might have no other choice. If I lived down the road from you I might have been able to help you out a little better.
Good luck to you and your machine