Some people on here hypothesised that it was simply a matter of it being inconvenient to start testing because of the other new trains in the pipeline. I've no idea if this is true but does anyone know what the holdup really was? Or was it just the wrong time to test? If they have fixed something they've done well to keep it under wraps because the vehicles are spread across several sites none of which are secret a la GCHQ.
I have no inside knowledge, so the following is just deduced from previous posts, by those who seemed more knowledgeable, in this very long thread. I cannot vouch for its accuracy.
When the first two units were delivered to Northern, a gauge clearance issue was discovered - part of the exhaust system fouled the lower sector structure gauge. After the exhaust plumbing was modified, it seems that the emissions testing on the engine installation had to be repeated before the units could be cleared to run on the main line. These Stage IIIB compliant engines rely on Selective Catalytic Reduction in the exhaust sytem to meet the emissions requirements, so presumably the modification must have invalidated the original testing. Apparently there are only a few specialist test houses equipped to carry out the tests and they had a big backlog, causing a delay of several months until a slot could be obtained. Possibly there might have also have been supply chain issues in fabrication of the new parts for retrofit and production.
There was a further suggestion upthread that there may have been brake system issues, due to the additional weight in the driving trailers. There was some brake testing carried out a couple of months ago, presumably successfully.