If some early rumours are to be believed, they were banking on 20nm processes being available but that process node turned out to be a dud for anything by LP processes.
Now, the issues with HBM weren't specified, so it could be problems with the interposer or even the memory itself. Intel, notably, was supposedly having yield issues with the crystalwell edram due to its constraints.
One other thing to consider is that the costs and research AMD has sunk in bringing HBM to market now may yield benefits on their next product which will be using HBM2. It could well be that Nvidia will run into similar issues with their HBM2 integration.
Their 14nm releases will be the Arctic Islands series (4xx), which will be based on GCN 2.0 as opposed to Fiji which is in all likelyhood GCN 1.2.