AMD plans to stick with 28nm throughout this year, only later will it move on to 20nm and FinFET after that. The main issue behind the delay seems to be TSMC's inability to deliver a mature 20nm process.
AMD and NVIDIA manufacture their GPUs on a common foundry, TSMC, which has faced delays in implementing its 20 nanometer silicon fab node transition, forcing both companies to come up with new GPUs on existing 28 nm nodes. A huge leap in performance could be a tough ask for those new GPUs. NVIDIA is expected to tape out its performance-segment GM204 and mid-range GM206 chips, both of which are 28 nm, later this month, and the first GeForce GTX products based on the two are expected to roll out by late-Q4 2014 and early-Q1 2015, respectively.