Apple will release the watch a bit later than it had hoped because of technology challenges. It probably didn’t help that several important employees jumped ship. Nest Labs, the smart appliance maker that was acquired by Google last year, poached a few engineers who were the very best on the watch team, according to two people. Among them was Bryan James, Apple’s former director of iPod software, who became a vice president for engineering at Nest in early 2014, these people said.