Hi have seen this happen a time or two for no apparent reason after working on the fuel system !! I connect a vacuum pump to one injector fitting leave the other three connected to the injectors , pump your lift pump while sucking vacuum through inj pipe from inj pump( I use inj pipe no1 ) if you start to get fuel from the pipe then remove vacuum and put inj feed pipe back onto injector , leave it slightly loose and crank engine over if it fires and starts then tighten up injector pipe !! Ive used this method a couple of times and found it to work great !!! Don't know the reason why the pump can do this other than maybe an airlock in the upper parts of the inj pump .. Good luck..