Hi balloons,
It sounds like you have set 'Main' as the Default Value for the dropdown field. This would mean that every time you create a new row on the Repeating Table it will set that Fields value to its Default.
Do you want that Field to appear blank by Default and then you choose the relevant option or are you looking the Field in the first row to be 'Main' with each new row after blank to allow a choice?
If you want the Field to remain blank for each row just create a new Value in the Dropdown Field Properties but leave the Value and Display Name blank. Then highlight that value and click Set As Default.
Hope this helps.