With my skill points, dex modifier, and tumble boots I get 10 to tumble but I don't get the +1 ac I should have. Is that a bug or am I missing something?

You get the bonus for every five ranks you take, not counting for example dexterity bonus. It's on the skill description.

Your armor and shield penalties also factor in. You'll need to take 5 ranks BEYOND whatever your total skill point penalty is before you see a AC increase.

Ok thanks.