Parallel and split options are always going to be lower output than full series humbucking.

As the switches and wiring for each switch should have been exactly the same, then I'd first suspect a dodgy switch or dodgy soldering.

A broken coil winding could also give you the same results so that you only get a sound in the parallel position. With a break, you won't get any sound in the series position and if the broken coil is the one selected by the split, then you'll no sound again. If the switch contacts all buzz out happily with a meter, then measure the resistance across the two coils and check you get a 3k-4k ohm reading for both coils. You'll really need to unsolder the wires from the switch to do that.

After that, there is a very small possibility that one pickup may have it's wire colours swapped, but you should be able to check this by comparing the underside of the two pickups. I can see from a photo that the green and red wires are connected to the same points, but can only see the black and white wire connections on one pickup.