If the needle is in properly, flat to the left, and is the correct 15x1 type,
and it's threaded properly, from right to left, then somethings amiss in the bobbin area.
If the bobbin case is in properly as in misskira's drawing,
and the bobbin itself is in the case properly, as in it rotates counterclockwise when you pull on the thread as the open side of the bobbin case is facing you,
then there is either some thread under the needle plate that's causing problems, or perhaps the timing is off. FWs can be knocked out of time if they are messed with.
A quick check is to remove the bobbin case, and run the needle down as far as it will go. Slowly bring it back up about 9/32" and see where the point of the hook is. It should be directly behind the needle at the eye. You've got some leeway, but not much.
Hope this helps.
Joe