it seems like all my words have maximum strength, although some words seem to slip my mind much more often then other, not to mention different forms of the words. duolingo should reflect this (either my mistakes or the amount of practice i had in each word) so to challenge me more when i wish to practice weakest words.
Currently, "word strength" is exclusively referring to recency. Not using a word (or not getting it right for a long time) is what decreases it's "strength". That's all it's intended to do.
That said, it would be nice to have a feature keeping track of what words you were getting wrong more often, and recency, though generally correlating with it, doesn't directly relate to a word's strength.