Is Scale set to 100%?
Is Enlarge/Reduced Printing off?
What value is Roll Paper Safety Margin set to?
You say you are not printing borderless. Does this mean that (as you have said) you print are just smaller than the paper, or do you mean specifically that Borderless is set OFF?
Is Fit Roll Paper Width set to OFF?
Are you using standard sizes when you set up the print job, or have you made Custom sizes?
If custom sizes can you make a new one, verifying in the print dialog you set all 4 borders to 0"
Is it possible that in your file prep there is a preset or action somehow being applied that adds a border (white) before final resizing. ie the print size being sent to your printer is correct but it has 1/8" white border around the image itself?
When did this problem start?