The tension on the bobbin (under the deck) needs to be right. When you load the bobbin, try and pull out the thread a little. It should be firm. You should have to pull a bit to get thread out. If not, the tensioner on the bobbin is set wrong or has backed off (Or you loaded the bobbin wrong) Don't use the bypass slot!
There is one screw on the side of the bobbin holder, where the thread is pulled through when you load the bobbin. This screw can be adjusted to control the tension. See pfaff page on bobbins