Top zigzag is fine, bottom is loose

SOURCE: Singer 301a...sews on top of fabric

This is a tension adjustment problem. It's good to use two different colors of thread, like you did. From your description, the top thread (needle thread) tension needs to be tighter. As you tighten the tension of the top thread, you'll see that the top thread pulls the bobbin thread up into the fabric so it's no longer laying loose on the bottom of the fabric.

You'll need to have the pressure foot down when you tighten the tension. Tighten it a little, sew a practice seam, check to see what the stitch looks like. Tighten it a little more, sew, check, etc etc--until both top and bottom look the same--you shouldn't see much (if any) of the bobbin thread color on top of the fabric or the top thread color on the underside.

Let me know how this works for you, ok?


Robbie

SOURCE: bottom thread too loose; tried

Check for correct bottom thread tension.
It's provided by a flat spring on the bobbin holder.
When the bobbin is correctly installed, pulling on the thread
should meet with slight resistance, and the bobbin should spin without
trying to jump out of the bobbin holder.

SOURCE: bobbin case on singer ce250 thread gets hung up and pulls bobbin case out of place

is the needle inserted correctly--flat side goes to the back of the machine
the possiblities are
worn or broken gear
machine could be out of time
needs to be serviced

SOURCE: My top thread (Needle) isn't grabbing the bobbin thread. Help

just adjust the needle bar down