Why am I finding hard spots in my yellow potatoes no matter how long I cook them or what cooking method I use? These are yellow flesh potatoes but are not labeled as "Yukon Gold". This has happened with the last two bags of potatoes I bought. There are areas that do not cook; when I bite them they are hard and crunchy like a raw potato.

The potatoes appear to be good; no green skin, no sign of damage, disease, or mold, no sprouting eyes, and these spots do not appear to be related to the places where eyes would sprout. I am about to stop buying yellow flesh varieties of potato altogether. This absolutely ruins mashed potatoes, and isn't appetizing in baked ones either.

From this description it sounds a lot like you have experienced the somewhat rare occurrence when a potato simply refuses to perform as expected. First, we are compelled to examine the science  what defines a potato. The industry measures potatoes in terms of specific gravity. In layman’s terms this is described as starch or simply, solids. For example, a typical Idaho russet potato consistently measures 21% solids - the remaining 79% being water, making them an excellent all-purpose dryer-fleshed potato. If this sounds like a lot of water, consider that the average fresh produce item is well over 90% - plus water content. Relatively speaking, consumers get more bang for their food dollar as potatoes’ yield after preparation offers more to consume than say, cooked- down greens. 

While russets have higher solids, colored varieties such as round whites or yellow types (including yellow types marketed as ‘Yukon’ or ‘Yukon Gold’) are classified as ‘waxy’ potatoes due to their lower starch/higher water content. These varieties have excellent properties for dishes such as potato salad or any recipe that calls for a softer texture or skin-on color appeal.  

But back to the issue at hand. Very occasionally (but one we hear about at least once or twice per storage season) a small pocket of potatoes in a given field (and a possible with any variety), for reasons unknown grow into an inert, higher-than-usual solid content. Normally, when a potato is heated the water molecules within react and ‘cook’ the potato. In the anomaly, very-high solid potato little water resides within, hence nothing to permeate around or within the solids’ cells to cook the potato, no matter how long exposed to surrounding, external heat medium such as baking or boiling - explaining the hard or crunchy potatoes texture described. Nature has her peculiarities in all commodities, the potato industry included.