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.

Click Save.

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.