Encumbrance Accounting Reverses?

Dear All Experts,

We are going to use Encumbrance Accounting for Purchasing and Financials. Here we are observing that while creating the Requisition and then PO, accounting is OK and also Budget is encumbered, but when we are going to complete all cycle like creates PO, receiving , Invoice & payments than again same budget is available for another requisition and PO. Why this happens , did we miss any setup?

Thanks in Advance..