Please provide more information. Usually, GPC is done with a bank of columns, sometimes 4 or 5. You can lower the number of columns if you use a mixed bed column. What columns did you use?
What molecular weight are you looking for? Also remember, there can be differences in the estimated molecular weight (by GPC) and the actual value.
Your pressure, flow, and mobile phase look ok.
Best company I've found for technical assistance on GPC is Waters. You may want to contact them.