This guide is based on the questions constructors have asked me over the years.
Does the gun fire when the trigger leads are shorted at the terminal block? If not then your trigger lead is faulty. If you are using a hot shoe lead with more than two wires coming out of it, make sure you connect to the correct two. Dont just guess - do a continuity check!
Have you used the correct link position for LK1?
Is the battery correctly inserted? If not then you may have damaged the unit
Have you cleaned the PCB properly?
Have you put the components in the right way round, in the correct places, and soldered all the joints properly.
You are using the incorrect position for the programming link, LK1.
The battery is dead, or not decoupled properly (check you have fitted C5 the correct way round).
The PCB is damp; you should conformally coat it.
Is the battery dead? Replace it if it reads below about 3.4V as it is, for practical purposes, dead at this voltage. The unit will operate down to about 2.6V or so with alkaline batteries.
You have conformally coated the PCB with an electrically incompatible compound - bad luck!
You have not properly cleaned the board after soldering.
You have damaged the photodiode or IC by not taking proper anti-static precautions.