Even if you pay it off every month?
The short answer is?
Most banks only report one time per month and while a few may report less often, it is very rare that they will report more often.
Therefore, your credit report is probably showing the large balance every month, thereby reducing your score because of the all-important debt to available credit ratio. Even though you pay it off every single month, the high balance is most likely being reported and showing on your report.
So while it is good for your own personal finances to pay off your balance every month, it is not good for your credit report, especially if you charge a large amount every month.