Wrong thread tension and issues in bobbin case makes the lower thread break easily. If this occurs, you may check your machine if one of the following happens:
Bobbin case is threaded wrong
It is important to thread the bobbin case in the right way. Check both the bobbin and bobbin case. You should slip the bobbin thread into the side slot of the bobbin case and then bring up to the needle. Check out the details of inserting the bobbin here.
Insert the bobbin in the case by passing the thread side slots.
The tension of lower thread is too tight
Like upper thread, lower thread breaks with accumulated strain when working with an excessively high tension. Lower the thread tension to see if the sewing improves.
Working with a suitable tension can prevent lower thread from breaking.