Bobbin Thread Showing on Top: Check Upper Drag and Bobbin Seating Before Touching the Bobbin Screw
Bobbin thread showing on top means the lower thread is being pulled farther toward the top surface than it should be for the seam you are trying to make. Before touching a bobbin-case screw, confirm that the visible thread is really the bobbin thread, then check upper-thread freedom, upper threading, bobbin orientation, and the documented