People usually face this problem that their bobbin thread starts showing on the top, the root cause of why it happens is tension. Well, in this guide it is explained which tension problem does it and how to solve it.
The thing which can cause the bobbin to show on top is the weight of the thread, if you use a heavy top thread or a very fine top thread then it can cause a misbalance between the two threads. The only solution is to change the top thread and replace it with a lighter one.
Usually, the thread gets caught around the base of the thread spool, in the worst cases, the thread gets looped and starts putting too much tension on the upper thread and causes the bobbin thread to show on top. So, you should check the thread spool if you face such a situation.
The tension discs located at the upper thread path get very dirty by the lint or pieces of broken threads due to this tension in the upper thread increases and the bobbin thread gets pulled on top. In this situation, you need to clean the upper thread path with a brush properly.
Whenever you use a sewing machine make sure you use the right case for bobbins, and that the weight and type of the bobbin thread are correct. If you use anything wrong then cause the bobbin thread to show on top.
The bobbin should be cleaned by removing it, make sure you use a brush to clean it. After cleaning the bobbin case and make sure you also clean all the lint underneath. It would be best to clean it after using the machine for 8 to 10 hours.
If all of the strategies fail then make sure you decrease the tension of the top thread. First, decrease tension slowly but if recognize that it's making a difference then decrease bigger.