Article is somewhat misleading. The software that implements the gun (Block 3F) is slotted for 2017. That is largely a program management decision and not a major "technical" snag.

The F-35 may not be able to fire its guns in operational service until 2017, but test pilots will likely fire the gun well before then. The F-35 is designed to use 25mm rounds instead of the more common 20mm rounds used currently in other jets including the F-22 Raptor. The 25mm rounds have to be physically certified, then the software for the gun validated and certified, and finally pilots (& ground crew) trained to operate the gun.