There are a couple of reasons why a store may not accept your printed coupon:
Low ink or toner in your printer can effect the quality of your printed coupons. When this happens, the bar code may not print properly and will likely be unscannable which is a valid reason for a store to not accept a printed coupon.
When your printer is set to a low resolution setting, the bar codes on your printed coupons may not print clearly and will likely be unscannable which is a valid reason for a store to not accept a printed coupon.
Most stores will have both of these points listed out in their coupon policy, so it is always a good idea to be familiar with your stores coupon redemption policy and ensure that both your ink/toner is full and your resolution settings set to normal.
However, if there is nothing visibly wrong with your coupon or bar code, and there is nothing preventing you from using printed coupons in the store's coupon policy, the decision to not accept the coupon likely falls to the individual store manager. In this case, we recommend that you fill out an in-store complaint as they are preventing you from saving money.