Withholding Tax on Down Payment

I have set a minimum amount for the tax code as Rs.10,000 and the formula is set as upto Rs. 10,000 nil & above Rs.10,000 @ 10%. I have created 2 WHT types - for payment & invoice. While entering down payment on say an amount of Rs. 4000, TDS is not deducted which is correct. However no WHT is deducted on subsequent down payment of Rs.7000. Tax should have been deducted on the entire amount of Rs.11000. I have set the Tax Type as with accumulation starting from period 4. Why is tax not being correctly deducted at the time of down payment?