Not sparking on one side

Hi I have a car that has had the engine rebuilt (to standard) since putting the engine in it won't start, have checked the fuel which is all good, I have also checked the spark, have found that cylinder 1&3 spark as it should but cylinder 2&4 will spark once at the start and not again.

I have since check the timing, found the timing marks to be spot on, I have change the NS cam sensor, still not working. I have now scoped the cam sensors, the both sensors are showing a good signal although the voltage of the signals are reversed which I find strange. (OS is image 1)

Can someone please help me locate this fault as I have run out of ideas. 

Thanks