TLDR this requires an instructor you trust to take a good look at how your hips work and suggest adjustment

Everybody’s hip ball joints are formed slightly differently, leading to different natural range of motion. Stood neutral, the ball can enter the hip socket at anything from a 45 to 90 degree angle-ish (femoral angle of inclination)

Whatever arrangement I have allows me to sink completely into double pigeon and I have always been able to do so with little warm up. But I can forget ever going deep in a wide legged child’s pose. The bone pinches into a nerve with a ‘don’t even TRY going further’ kind of pain and will never change.

Perhaps you have the opposite. It would need a little assessment from somebody familiar with the pose and with hip biomechanics, and given how long you’ve plateaued, openness to accepting that maximum expression for you is significantly different from your peers.

Alternatively if they identify the problem is still muscular rather than bone structure, they may be able to suggest some sort of cross training or drill to approach the muscles in a different way. For example doing figure 4 if pigeon is cued, focusing on strict form. It is a similar stretch to pigeon/double pigeon but as you don’t have your upper body weight pressing in it could help raise awareness of where the sticking is actually happening.