The hard limit for hearts is 3 hearts within 18.75 blocks, 6 within 50 blocks, and 10 for the entire world. So if you say killed off a boss that drops a lot of hearts (BoC, EoW, Destroyer, WoF) and didn't pick up all the hearts than they can easily cap out the limit and produce nothing else. I honestly find it much easier to wire together 3 (at max) heart statues and have them keyed to a couple pressure plates, one on each side. That way you can land and run across for a quick heal from any direction.