Tabing issue in combo box

Hi all

As part of the process to create a user friendly call tracker - we have come this far ( check the attached sample tracker)

One feature enhancement to this tracker was to make it easier for the users by incorporation of combobox instead of list boxes to facilitate auto fill.

Now we are experiencing a unique problem with this enhancement ( any suggestion \ tips welcome)

Observation

1) After we converted the sections like "caller designation " etc to combo box the auto fill is happening but the Tab function ( that is use of tabs to move around the cells is not working)

I also noticed that if you tab into a combo box the auto fill does not work - but if you click and enter a Combo box the auto fill works ( click refers to click the cell and not pulling down the drop down menu)

any suggestion why this is happening and any solutions to rectify the same

Thanks