Why is Labour struggling?

Short answer: The Greens.
This is not to say that the Greens have been doing anything wrong. On the contrary. Going by consistent opinion polls, they stand to improve on their 2011 party-vote result (which was 11%). They play by the rules (unlike some). Their leaders are effective. And they have a clear and well-developed manifesto. Consequently, they are more assertively office-seeking in their pre-electoral rhetoric than before.
And I'm not saying that Labour has got it wrong either. Their leader is relaxing into the role better than expected (despite minor glitches). They too have a credible alternative policy programme to offer. Parker and Robertson make a great back-up for Cunliffe. But Labour's opinion-poll results have trended downwards since late 2013. Do you remember those heady days when Labour was polling in the low- to mid-30s? It looked like Labour plus the Greens could seriously challenge the government. So, what went wrong?
The problem is that people have figured out that a vote for Labour is also a vote for the Greens. Now, many Labour-voters are happy with that. They look forward to such a coalition and the balance of forces that it would bring.
But many would-be Labour-voters are disenchanted by the prospect. Add in Labour's probable need to rely upon Winston Peters in any governing arrangement, and voters are even more put off. Hence, they have either decided not to vote, or they have migrated to 'the devil you know.'
At least Cunliffe has been transparent in telling us with whom he would be prepared to form a government (Labour–Green–NZF). But it's not a recipe that works well for sufficient numbers of voters. In effect, the Greens' success as a party-seriously-seeking-office means defeat for the left as a whole. Better luck next time.