Is the humanity among your consumables, or in the counter next to your life bar?. If it's the first, then you need to use it as any other item, after that the counter next to lifebar will raise by 1 point that means you are "holding" the humanity and now you can reverse. The only other circumstance i can think of is that you're cursed, but in that case you would only have half health bar, and a skull in the counter where humanity goes.

There's 2 types of humanity:

1. humanity resource- the amount of humanity resource you have. It's a double-digit counter next to your life/stamina.

2. Humanity - an item, looks like a small black sprite. Doesn't do anything until you use it. Using it gives you 1 humanity resource.

You probably have 5 humanity items. Use 1 to actually give yourself 1 humanity (you'll see the counter next to your life/stamina go up by 1). Then you can reverse hollow at a bonfire.