I have an older Singer Model 9134. I was sewing a quilt and all of a sudden it started skipping stitches then stopped picking up the bobbin thread all together. I took the plates off and looked at the hook mechanism and what is happening is when there's fabric in the machine, the hook that rotates around the bobbin casing is not picking up the top thread. However, when there is no fabric in the machine, it will pick up the top thread. That's what has me stumped. I've already tried changing the needle and playing around with the tension.

Any suggestions?