Is this an old singer featherweight or the new one?

If it is a new one it could be that the quality of "new" singers isn't that of "old" ones. I'd think about returning the machine to the dealer.
Susan

It's an old singer featherweight.

You might need to adjust the tension in your bobbin case. There is a little screw on the case that holds the bobbin; with a small screwdriver, loosen it just a hair, then put the bobbin back in and try it again. Repeat if needed. Only a very tiny adjustment is needed until the stitches underneath look as even as the top stitches.

I had to do this to my FW but it's drummed into us to "never fool with the bobbin tension!" But that's what that tiny screw is there for.

Hope this helps!
Teresa

I agree with what Teresa says. As you sew it could be jaring the bobbin casing some. I know that the upper tension springs can become "sprung" over time too.
Susan