When the stitches become loose, the thread cannot piece the fabrics firmly. If you cannot sew a completed good due to the loose stitches, you may check if one of the following happens:
The machine is not correctly threaded
If the machine is threaded wrong, not only does it make the thread breaks easier, but is also more likely to create loose stitches. Check the threading to see if the thread has passed through the entire thread guides, the take-up lever and the eye of the needle. If not, re-thread as instructed in the user manual. Check out the details of threading the machine here.
The thread should pass through all parts of thread guide and take-up lever.
The bobbin case is not correctly threaded
When sewing, each stitch is formed by the upper thread and the lower thread. If any one of them is incorrectly threaded, the stitch does not form well. Therefore, make sure to check if the bobbin case is threaded in the right way as illustrated in manual, and re-thread if needed. Check out the details of inserting the bobbin here.
Insert the bobbin in the case by passing the thread side slots.
Needle/fabric/thread combination is wrong
When sewing with different types of fabric and thread, it is important to change needles accordingly, or it may not be able to sew the fabric, or even damage it. The size of the needle should be strong enough to pass through the fabric, but fine enough to minimize the impact to it.
Working with a suitable needle/thread/fabric combination can avoid loose stitches.
The thread tension is wrong
A low tension makes the thread unable to be sewn through and form firm stitches to piece the fabric. Re-adjust the tension to see if the issue improves. A suitable level of tension should sew the fabric smoothly, without any puckering and easily removable stitches.
Working with a suitable tension can avoid loose stitches.