If it's only a few spots, I have marked the skip with a pin and come back and fix them by hand later. I put one strand of the same coloured thread in a hand needle. I pop the knot on the end into the fabric and carefully come up on one side of the long stitch. I then go down on the other side of the long stitch being careful to come out on the backside of the quilt right next to the long stitch there. I then go to the other side of the thread and insert the needle beside the long stitch thread and either bury the knot or proceed to the next stitch. Basically I make it look like the stitch would have looked if they were connected by the machine. This is only practical if you only have a few spots to fix and it really bugs you.