Stress pattern changes with noun and verb homographs: exceptions to the rule

We all know that words that can be used both as nouns and verbs have a different stress pattern: http://www.wordstress.info/wp-content/uploads/2014/10/Stress-Pattern-Change-noun-verb-pairs.pdf

As a non-native english speaker, I love rules. And, just like a young child, I make some (funny) mistakes when it comes to the exceptions. For instance, "award":

Because of the above-mentioned rule, I automatically pronounce the noun as "A-ward" and the verb as "a-WARd". I know that there is no "A-ward", but that's precisely my question:

Why do some pairs of verb/noun homographs not follow the stress change rules?

The only "rule" I know of about this is of no use predicting the stress of a noun from that of a corresponding verb, or the other way around. It's this: when the stress of a noun and corresponding verb differ, either the noun will have primary stress on an earlier syllable than the verb has or the verb will have a syllable stressed which is unstressed in the corresponding noun.