Problem logging on to Navision Web service from Windows mobile\.net compact

I have a problem logging in to web services from windows mobile, i.e. .net compact.

The problem is when I log in, it SOMETIMES gives me invalid userID and Password error, despite the fact I know them to be correct.


Being Window Mobile, i.e. .net Compact 3.5 I cannot use functions like usedefaultcredentials, so use the following:


POService =

new SG1.PVPurchOrder.DynamicsNAVPVPurchOrderWMS_Service();

POService.Credentials = 

new NetworkCredential(this.sUserName, this.sPassword, this .sDomain.Trim());


When I come to read a PO I know is there I use :



this.poSelectedPO = this.POService.Read(txtPONumber.Text.Trim());


but it SOMETIMES gives me invalid userid as error.


What am I doing wrong?