Hi Adrian,
Your Sales Reps should be getting synced from Method to QuickBooks, are you linking the Sales Rep initials to a "Name" that is stored in your QuickBooks File? I just did a quick test in my own account, first I added a Sales Rep with a 'name' from Method, then went into QuickBooks, pulled up an Invoice and looked at the Sales Rep dropdown, my entry was in the list. Then, I went back to Method, created another Sales Rep by adding their initials, but not linking them to a 'name' in your QuickBooks file (name meaning Vendor/Employee). This time when I went to check for my new Sales Rep in QuickBooks, it didn't show up. At this point, I checked out my conflicts in Method, QuickBooks->Resolve Conflicts, here I saw I had just created one conflict for a Sales Rep.
Here is my conflict:
"The Sales Rep could not be added for RecordID '26', since there appears to be no Entity (Vendor,Employee or OtherName) assigned."
So, even though Method doesn't require you to link a Sales Rep to an Employee/Vendor, QuickBooks does, so QuickBooks will reject any Sales Reps created in Method that don't have a Vendor/Employee attached to them.
I am not sure this is the issue you are running into, but I am not sure many other reasons why your Sales Rep won't show up in QuickBooks after entered in Method. If this isn't your issue, double check your account is update to date with its sync, and make sure you don't have any conflicts for this record.
-Ben