I agree with others that this looks like a tension issue, not knots.
If that photo is the back of the two fabrics, then those little "bumps" at each stitch are the links where the two threads, top and bobbin, link together. Those links should be inbetween the fabrics being joined, and not visible from either side. Since the "bumps" are on the backside of the joined fabrics, that is top thread which is being pulled down too far by the bobbin thread. If you can increase the tension of the top thread (do it in small increments, testing each time), then those bumps should disappear once the tensions between top and bobbin threads are equal.