Vendors due date is not popping up as per terms of payment

Hi In AP the vendors due date is not popping up properly as per invoice dates. Suppose the terms of payment of vendor X is 21 days then the due date should fall exactly after 21 days from invoice date but it is not happening here. I checked in terms of payment setup tab there at days field 21 is mentioned even though its not working. And one more thing is for few invoices the due date is falling correctly but not for all the invoices. What could be the possible reason?

Can you post the payment terms calculation?

Hi Adam the following data shows how due dates are falling. Here the terms of payment of vendor is 150days but no where I find exact due date. For this vendor not even one due date is coming correctly.

Voucher Invoice date Due date

BLR/PR/1213/0318 12/12/2012 11/5/2013
BLR/PR/1213/0321 17/12/2012 5/5/2013
BLR/PR/1213/0339 28/12/2012 5/9/2013
BLR/PR/1213/0346 3/1/2013 29/04/2013
BLR/PR/1213/0350 4/1/2013 25/05/2013
BLR/PR/1213/0356 16/01/2013 13/05/2013
BLR/PR/1213/0357 17/01/2013 9/6/2013
BLR/PR/1213/0362 19/01/2013 7/11/2013
BLR/PR/1213/0365 23/01/2013 13/11/2013
BLR/PR/1213/0373 28/01/2013 16/06/2013
BLR/PR/1213/0375 31/01/2013 23/06/2013
BLR/PR/1213/0379 6/2/2013 22/11/2013
BLR/PR/1213/0380 11/2/2013 30/06/2013

What is the payment method in the terms of payment. Update it to Net and check again.

The payment method is Net. What could be the root cause of this issue.?

Can you post a screen shot of the payment terms and the calculation.