The issue might be with the credit card processor. Typically the way it works is when you initially charge the bill the card processor with authorize the amount

The issue might be with the credit card processor. Typically the way it works is when you initially charge the bill the card processor with authorize the amount of the meal plus some percentage (Pre-Auth - I forget the amount or it may vary) and then after the bill is signed you do a second transaction (post-auth) to record the final amount being charged and this releases any difference. Depending on the card processor it may not be possible to post-auth more then a certain percentage of tip because it exceeds the amount authorized and/or they are flagging tips beyond a certain percentage as possible data entry errors and declining so that it does not result in a chargeback. So if you want to tip your waiter $1200 for a $20 meal, you ought to plan on doing it with cash.