So it's skipping stitches, suggesting that a timing setting is off. Usually it's just the needle/hook distance. You can check that by removing the needle plate, set the machine to the widest zig-zag, turn the handwheel until the needle is on it's way up on the left side. Is the space between the hook and the needle wider than 1 thread? Check by pressing the needle toward the hook. If so, we need to move the needle bar towards the hook.
While you've got the needle in that same position, is the hook coming across the needle, just above the eye? That's where it should be. If not, you may need to raise or lower the needle bar.
Let me know what you figure out.