Frank Forencich wrote a bit about stretching and strength that I remember. The idea was that it may not be a good idea to stretch too much and artificially increase your flexibility, and that tightness may in fact just be a protective mechanism to ensure that you don’t outpace your muscular capacity and endanger your tendons.
Ironically then, doing strength training can signal to the body that it can safely increase range of motion without risking injury.
As an anecdote, at this workshop I attended with him, we did some sort of yoga/pilates/feldenkrias method type thing which was supposedly strength building. After doing this, I found that I actually lost flexibility in my typically gumby-like limbs, and wondered whether that was in fact a good thing. Maybe I was more attuned to my signals and not risking injury through over-riding the failsafe.
I dunno if that helps, though. Maybe try spending time exercising the range of motion you want and see if it expands?