Most likely you are only getting a signal in passive mode, which is one of the two positions of that switch. This can be proven by checking for continuity between the pickup signal hot and the central wiper tag of the 500k half (upper, at least in the diagram) of the volume pot. If this is the case, then fault-finding quickly becomes very complex with the active side of the circuit. I'd first trace the signal from the pickup as it comes to the switch pole, out to the relevant line on the preamp board which looks like the far left white wire. That lets you know the preamp is getting signal.
Also check that the preamp is getting power, which should be +9v, 0v and -9v over two series PP3 batteries with a central tap being that virtual ground of 0v. Measuring from the union between the two batteries (0v), measure the voltage at the preamp with the long connector. The first one (blue) should be -9v. The third (red) should be +9v. You should also have continuity between the junction and the fifth wire (black) which is the 0v, or "earth" in that diagram.