Okay the run down for a VS machine........
Check to see if your bobbin is inserted right and not binding inside of the shuttle. Then see if the shuttle has any abnormal wear on it. I had a 27 do the same thing yours was doing and the shuttle was worn slap out. I replaced it and it works like it should. You should oil the shuttle and the radius that it rides in that is where the wear comes in to play. Maybe the needle moved a tad and is not seated all the way up in the holder or the needle might need to come down a bit. With the shuttle plates off of the machine and shuttle installed turn the hand wheel and watch what happens. Is the shuttle passing the needle at the right time? Is the shuttle going through the loop the thread is making, and is the thread making a loop for the shuttle to go through. Is the tension on the top thread to tight?
Check these out to make sure that its working properly and adjust as needed. Also this might seem really stupid but is the machine going in the right direction?
Billy