Google claims due to a resource constraints, they won't allow many very long tail KWs to be served, and thus render them inactive based on a history of having never before been typed. There is a threshold, which they won't reveal, that needs to be met before they will activate these keywords.

Resources be damned, so essentially Google is preventing long tail phrases from showing - why? Really it is because there will be no competition on them, and that makes less money for them. So they force long phrases to map top medium tail words, where there is more competition. But of course, whereas you would have gotten a 50% CTR and an amazing QS, not you get lower all around due to competition on the SERP.

I've taken 3 concepts (mens + color + apparel item) and concatenated in all configurations, but many end up inactive due to no history. "Green mans pants" for instance. Instead traffic goes to my "mans pants" phrase-match keyword, and eventually that phrase get's overloaded and can't handle everything.

Damn thresholds.