The singer 6212c takes a class 66 bobbin. I have one of these machines myself, it's from the '80's and takes the same bobbin as my 1913 66 red-eye treadle. Enjoy, these are some of the easies to find bobbins.
Either the bobbin has been incorrectly wound, installed incorrectly, and the upper thread hasn't been fed correctly. Sometimes the wheel to feed the upper thread is moved in the wrong direction.
I have the same model. I'm not sure EXACTLY when it was released, but the owner's manual (I assume the one I have was the original) says it was published in 1984. I hope that helps any-it's the closest I can find. I received mine for Christmas in 1984, so it was probably made that year.