I've been focusing on low HR training for ~15 months now and similar to you am 41yo and aim has been to have HR on easy runs between 130-135bpm so have a few thoughts. There's the obvious ones like HR is impacted by external factors (weather, elevation of run, etc), but I would also say it does take time. I had to really disregard the ego when I switched over; I used to be running 7:45/mile for my easy runs, when I first started I was instead running 9:10/mile. Over time that gradually dropped down to 8:40/mile and in the past month have been regularly running 8:10/mile on my easy runs. It takes a lot of patience but I genuinely believe it means I'm a) building a bigger aerobic base and b) it makes sure that I run my easy runs easy which can only help with recovery.
Hope that helps.