Plant establishment time seems to make a difference.
Plants that have shipped in sometimes don't have as much strength until they've been here awhile.
I have some old established plants from the early 90's and see that there are strength differences in them also.
Since I'm new to hybridizing, I purchase plants wherever/if I can afford them..
Last month I got Curt Hanson's "Women Seeking Men" from a lady up North.
She was hesitant about shipping it to me since I live in the South and she didn't know if it would do okay here..
WSM is doing great.
I don't care about intro year; just strength and if plant features will get me where I want to go.
I also got some newer intros this year --- one is pretty strong
and the other which I expected to be stronger isn't showing vigor expected, yet.
Newer plants I got from Tink came in strong and stayed that way.
So how they are started helps too.