Aye the luck with xpolly for mutants is very very random ...
I also can go long without one mutation and then sudden get 2 on 25 plants.
That's RNG for ya ...
Like I usually get 6-7 SoRs on 25 plants ... that's like 1 on 3 ...
After months of trying I finally mutate a white bonsai ... figures, taking the seeds off this one, I got 2 SoR !!!
As for the B&W seeds, I just rely on the quest, I do 5 each day and am happy if I only get 2 fire ...
I do not rely on mutations anymore because it is just too darn random.