If you are loading your Accounts Excel file with a column for the SalesRepID indicating the sales rep that should be assigned to each Account
and the Accounts are not being assigned properly you need to do the following:
Go to the sales rep user that is not getting assigned his/her accounts properly: Setup -> Settings -> Users -> All Users and click Edit (pencil icon) on the user you wish to Edit.
Un-assign the user from all Accounts manually as follows:
Click Edit and de-select all of the Accounts.
Now, re-load your Accounts Excel file that contains the sales reps assignments.
Sync SupeRep app and the Accounts will show for the sales rep.