The SDB does not supply power to the flipper coils, it controls a path to ground. The power chain looks like this:
Rectifier board => flipper coils (always have power) => J1 pins 8 & 9 (always have power) => flipper relay (on/off switch for circuit) => J2 pins 1 & 2 (power during game, no power in attract or tilt) => flipper switches (power on one switch blade during game) => ground
So with a game started, the flipper relay will be engaged and provide continuity between J1 and J2. You should see power on J1, J2 and one blade of each flipper switch.
Since you don't see power on J2, check J1 as stated above. If you have power on J1 then you either have a bad flipper relay or a bad connector at J1 or J2 or cracked solder joints on the header pins.
If you have no power on J1, I'm guessing there's a broken wire on one of the flipper coils. The power comes from the rectifier board to one end of the fuse holder under the playfield. Another wire runs from the same end of the fuse holder to one of the flipper coils. Depending on the game it could be the right or left. Let's assume the right. Power is then daisy chained from the right coil to the left coil, so you also have to check the right flipper to see if there's a broken wire.