WHTax not deducted for one vendor

Hi

There is WHT Code: PA.

Its given in two vendors i.e. A and B. Liable tick is there for both the vendors for WHT Code PA

Now when i did F-48 (vendor advance), it deducted WHTax for Vendor B but not for Vendor A

No exemption certificate is given to any vendor. I checked the WHT Tab for both the vendors, its same. Then why its not deducting for one vendor and its deducting for one vendor.