If someone comes up with a good "out of the box" solution to improve this, please message it to me. My daughter is extremely inflexible and the stronger and more muscular she gets, the worse it gets. She is equal opportunity inflexible too, legs, shoulders, back, feet (toe point)... The problem is she does stretch every day at home. She does all three splits and sits in them for 3 minutes as that was what a ballet teacher once told me. She then moves to "oversplits" though she makes no progress on them.
It's like her muscles fight the stretching, literally. At the gym when she's with a coach they will often apply pressure and try to force her further, but they know what they are doing and I don't so I don't ever help her at home for fear of hurting her.
The lack of flexiblity though affects her leaps, her beam (can't get shoulders open enough when doing her bhs causing much aggravation with one coach in particular despite the physical limitation), her bars (straddle glide kip out of clear hip is hard because her straddle is so small)...
Clearly inflexibility and gymnastics don't mix, but she loves this sport...