Purchase Tax Posting to Work In Progress Account

This is a problem i am facing with drop ship PO assigned to project and cost category when transferred to invoice. The tax is not defaulting to tax account set up on tax details but to WIP account for drop ship purchase orders when assigned to project and cost category and transferred to invoices. the strange thing is that if i used the purchase order as standard and received/ invoiced the PO the amount of tax will go to the account set up on tax details setup. i am using dynamics gp 2010. can anyone please help!!