Changing a format does not affect how something is stored. Open the table in design view and remove any formatting from the field in question. Save and close. Open the table. What is the stored value? If the value is 17, then that explains what happened since in reality, the value of a percent is normally less than 1 so 17% in math is .17. So, first, you need to update the actual stored values. Run an update query that divides the values by 100. Once that is done, you can change the format to Percent and the .17 will now show as 17%.