I suspect that's intentional. Pretty much every political analyst says that one of the major reason for labour losing the last election was economical credibility. That being the case they want McDonnell front and centre rather than Corbyn, who doesn't really any reputation in this regard and whose style doesn't really suit debates on the economy.

if the spotlight shifted to things more along the lines of social policy and political integrity you'd see more of Corbyn and less of McDonnell.

The idea Corbyn must be the labour spokesman on any given topic at all doesn't really make all that much sense. You don't expect a conductor to be able to play all the instruments in an orchestra...

Not that I'd disagree that labour have been failing to take opportunities that have been presented to them. Just that it doesn't really depend entirely on Corbyn to do so...

I totally agree, but Corbyn isn't even registering at PMQs when presented with the likes of IDS and the budget. If nothing else, he needs to say Labour couldn't have prevented a global crash and match the inevitable austerity retort with 'you've missed every one of your own targets'. It's time to build a simple narrative of his own, cos the Tories don't even need new material at this point.