If you find the bobbin thread tangled round the bobbin, a simple fix is to cut a circular piece of cotton fabric small enough to fit into the casing (with a hole in the middle for the post) like a washer, then put the bobbin in. This should stop any backlashing.
If it is just the top thread loose underneath it is your top tension, and you will probably have to adjust it for different thread thicknesses. I always use threads on a cone, anything from monopoly, embroidery, polyester and cotton, and must test every change, finer threads need a looser tension.
Hope I have helped some