I don't think they said "zero chemistry." They just said that the romance had been put in at the network's request, and once they changed networks and weren't subject to that note anymore, they realized it wasn't a storyline that had developed organically. Now, maybe they meant zero chemistry and were putting it diplomatically, I dunno, but all they said was that they didn't think it was the right direction for the characters. I suspect it had more to do with the writers wanting to focus on Kara's exploration of her development as a superhero and a professional and feeling that an obligatory romance subplot was a distraction.
Yes, they didn't say zero chemistry. That are just my words. But the way they abruptly and awkwardly got rid of their relationship despite building up towards it for so long indicates to me, that they think they weren't working together at all. When they moved to the CW and the pressure from CBS to keep them together was gone, they dropped them like a hot potato. They didn't even bother to show them together for a few episodes and then break them up. They got rid of them as fast as they could. That is why I think they agreed with me, that those two have zero chemistry and there is nothing they could do, to make those two work together.
I don't buy it, that it was because they wanted to focus more on Kara's development as a superhero. They did this also in season 1, but still found time for Kara/James. In addition to that it is not, like Kara doesn't have a romantic subplot going on right now. For quite some episodes we have the "will they or won't they" storyline with her and Mon-El. They even kissed already.
Mon-El comes off as another on a long line of cookie-cutter/metro CW casting types with no depth (no matter how the story tries to apply it), as opposed to James who--early on--had a natural chemistry with Kara--and centered her, instead of tripped over himself trying to serve her / begged for attention like Winn. Mon-El is the metro version of Winn, just remove the mountain of insecurities. It is no wonder that SG/Kara's strongest character moment were in exchanges with James. He feels like a real person and not fanservice.
Mon-El is for sure not the most innovative character ever, but he is at least fun. James is boring, far worse written than Mon-El and played by a less charismatic actor, too. And it is of course subjective, but I felt no romantic chemistry between Kara and James at all.
It also doesn't help for my enjoyment of James in general, that he gets one crap and badly written storyline after another. In season 1 the Kara/James/Lucy love triangle story did him no favours. And now in season 2 there is the badly written Guardian storyline, which came out of the blue and just doesn't fit at all in the show. Arrow is the proper place for vigilantes, not Supergirl. If he wants to fight bad guys physically, why isn't he simply joining the DEO or become a cop? In contrast to Arrow's Star City, National City doesn't seem to be full of corrupt civil servants. He could do good like Alex and Maggie. They also made him the CEO of CatCo and he could do so much good with the influence that job brings. Cat after all did exactly that in season 1. But he acts like doing good with only words is beneath him and worthless. He had to be a hero in a costume like Superman and Supergirl, because he had a massive inferiority complex in regards to them and wanted to elevate his ego. So he goes around punching bad guys now, which he is miraculous good at despite massively lacking experience and training.