One thing I do to bury a knot is: pull the thread with the knot taut to where it will be going into the sandwich. Put your needle in just below the knot. Separate the threads a little with the needle tip creating a little space for the knot to go through. Pull the knot through and before trimming the thread, use the needle tip again to realign the fabric threads back to close the hole. If all goes well the knot is buried and you can clip the extra thread tail off.