This might sound silly, and certainly no fun, but make sure they are activating. I trained my hamstrings for years and must have been moving the weight mostly with my lower back and momentum (curls), and extremely quad dominant when squatting/pressing. Finally able to activate and make them grow by doing single leg curls with almost no weight. Or maybe even start with a resistance band. One leg at a time, and I would even "lose" control of the weight in the beginning, stop and hold it in place, maybe drop back a little to reactivate the ham, then continue to curl, squeeze at the top. It's slow and tedious, but if those muscles have been asleep for years, it takes some time to wake them up. Once you start using the hamstrings and they strengthen a bit, then you can start to squeeze the glute to further isolate the hamstring, and before long your ham will feel like your bicep feels during and after doing biceps curls...like it's pumping up. Then you will surely see a change in size and hang etc. if poor or little activation is the problem.