cache problem

Hello Guys,
In my site when customer complete their order, product quantity is deducted from database but in product page it shows the same as before. For example, product quantity is 1, status shows “Available in stock”. Customer bought that product so quantity is 0, status should be shown “out of stock” but it still shows “Available in stock” and database shows quantity is 0. When I do purge all then it shows expected status “out of stock” in my desktop. But in other device it shows “Available in stock”. do you guess why this type of problem arising. do you have any idea how can I solve it.