Just to recap...
Logic suggests that if the pump works when other taps are used, that proves the pump and the bulk of the wiring, so if its only one tap that's not working, then it strongly points to the either the micro switch in that tap or possible the wiring to just that tap.
You have already changed the microswitch, so now assuming you have replaced the micro switch correctly points to the wiring to that particular switch. Just in case you didn't know all the tap switches are connected in parallel, so any tap will complete the power circuit for the pump.
As JCL has suggested if you can short put the two wires at the microswitch that should operate the pump. If it doesn't then its must be a fault on that pair of wires. You will need to trace the wires back to where it connects to the the other switch wires.
If you don't feel confident in doing wiring then you will need to get hands on help.