immobiliser problem

Hi everyone ,my 2011 spark developed an immobiliser problem, an auto locksmith suggested a new key which did not solve the issue (immobiliser light stays red and car will turn over but not start)!
Having read about a similar issue on this forums purchased from a running donor car an ecu,transponder,keys,locks,and 2 immobiliser boxes, having fitted all to my car the immobiliser light still stays on and the car turns over but won't start.
I called an auto electrician out, he spent an hour with the car using various code readers and checking fuses etch says the ecu and key etc, all seems to be ok and thinks theres a separate wiring fault, the fuel pump comes on and the central locking works off the key.
I am at a standstill now with my car ,I can't afford to replace it and its too good to scrap, low mileage etc, any suggestions or advice would be greatly appreciated thanks