POs over Webservices - Tax on Expense Lines

Calling Webservice gurus....

We're trying to manipulate a PO we've created over webservices, specifically to add an expense line in.

We're using WSDL 2.5, and the sample code below..

//Start Code
//The following code does not seem to set the Tax Amount!!

PurchaseOrderExpense netSuiteExpense = new PurchaseOrderExpense();

netSuiteExpense.account = accountRef;
netSuiteExpense.amount = amount;
netSuiteExpense.amountSpecified = true;
netSuiteExpense.memo = description;
netSuiteExpense.taxCode = taxCodeRef;

//End Code

We can add the line, but for some reason don't seem able to put an entry in the tax amount.

We can set set the tax code, and purchase amount but this doesn't update the tax amount automatically.