First, are you sure you have more than 16 units in the province you start a battle (not counting artillery)?
2nd, before you call for reinforcement, make sure at least one entire unit (to the last men) left battlefield. If you're not sure, count units' icons at the bottom of the screen. If less than 16, you're good to go.
When all conditions are met the reinforcement button will be displayed in color. Otherwise, it'll be grayed out.
That's all. If it doesn't work, I don't know how to help you.
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