Problem with combobox

On my login form I have a combobox where a customer has to be selected. 
The user may or may not have access rights to the customer's data. These 
rights have been saved in a database table. If the user has access 
rights, all is well. However, if he tries to log in with a customer for 
which he has no access rights, he gets an error message. This is alright 
too. Now if he consequently changes to another entry of the combobox, 
the SelectedIndex/SelectedValue don't get updated and the database query 
  now runs with the wrong customer ID (that of the first, unsuccessful 
attempt), which obviously results in the user not being granted access. 
I have tried all and sundry but cannot figure out how to get at the 
correct SelectedValue. As a matter of fact, after the second attempt 
(i.e. that with a valid customer but with a wrong SelectedValue in the 
corresponding query) the combobox once again displays the value of the 
previous customer.

The databinding of the combobx is set up as follows:

   with combobox1 do
   begin
     DataBindings.Add('SelectedValue', dataview1, compID);
     DisplayMember := compSign;
     ValueMember := compID;
     DataSource := ds.Tables['Company'];
   end;

The query is

db.get_dataset(ds,'Select * from tbl_sysusermember where fld_user = ' +
     ds.tables['Benutzer'].rows[0].item['usernmb'].ToString +
     ' and fld_company = ' + combobox1.SelectedValue.ToString ,da, 
'Rechte',false);

Any ideas?

TIA.