The same thing happened to me and closing and re-opening the menu didn't change anything. I have +10 on my gloves and sword, so I suspect that health bonuses simply do not stack.
Maybe they want to encourage you to diversify or something, but it also means you must constantly destroy redundant gear.
On second thought... Now suddenly it works. I'm clueless... Maybe certain bonuses stack, others don't. But you must exit then menu first, then do something (roll, strike, whatwver) then enter the menu.