Hemlocks are special in that they don't need a lot of fertilizer. Once a year with something balanced is enough. Never is also enough.
At what height are the yellow patches? How deep was the snow last winter? I have a neighbor from the midwest. She told me a story about how her trees always got dead spots 5 ft off the ground. She eventually figured out, during the winter, when there was snow on the ground, it was exactly the level her dog would hit on his daily jaunts around the yard.
Considering how the rest of the branches look, I don't think you have a problem.
