So my question is, what do you expect out of a note. Seems to me it should be constant from the initial stroke thru the sustain. Or is it the variance that gives it life.
I don't think the variance does, but it may be part of their life when you find it musically inspiring. In a sense you can possibly breathe life into it. That's what the "music" is all about IMPO. However, by any chance do you have them restrung properly? It's worth checking. "Restring Guitar - Ben Woods Flamenco Guitar Techniques" on YouTube tells you the proper ways (as it covers some options) to restring.
As I see the original subject having been discussed quite technically or professionally, I would like to share my tip that caters more to you if you are not professionally performing.
You do not even have to change plain nylon strings if:
A) No damage is found as you inspect, and;
B) You can tune them up with reasonably accurate 8ve pitch.
You can extend the life of the wounds by reinstalling the strings in the way that the strip between the nut and bridge is shifted about 1cm. You'll be amazed how the lost high-end frequency is reinstalled (not fully though...). You can keep doing this as long as the strings are long enough for you to do so, AND again, if you find no damage (that you may often find after a while, especially when you use a capo for flamenco).
Better yet, you can re-install the wounds reversed: Put the saddle side end to the tuner, and tuner side end to the saddle. And then again you can start shifting 1cm every time you feel they have become dull.
I reiterate, the aforesaid practice must NOT be applied to the sets you play a concert with, but can only be applicable to the other sets you use for daily bedroom playing to prolong the life of the string while keeping sufficiently crisp wounds for learning and playing experience. Also, please note that the shifted/inverted wounds do sound significantly crispier than before, but will not be as beefy as a new set.
There is no risk in trying, so you can try and see and decide. I'd be happy if you like the results while saving some costs and getting a little greener.