HI,
reasons for not checking the budget may be many.
Out of which few are listed ,which I suppose you should check at your end
Check config for the following.
Check the budget profile and the availability control activation
Check the tolerance limits whether they are not more than 100% and you have set the indicator for error and not for warning
Check the availability control checking scope ,you have marked "X check "for purchase orders
other one is to check the document type for Purchase orders
check with the MM person too whether he has to apply any note or validation.
hope this shall help you and solve your issue