It happens. Did you let your server know that you were using your DP prior to him giving you your bill?
Here's why I ask: when a server prints a check, 2 copies of the check are printed. One copy is for a guest using a DP, there is a place to write in a tip. Typically a guest will write in a $ amount or write CASH. The server then runs your DP, charges the tip however you wanted to and returns with another copy for you to sign. Transaction complete.
If a guest isn't using a DP or doesn't let the server know they are using a DP, the guest is given a different type of bill. This one has no tip line, looks like a normal bill in a non-WDW restaurant. If you have only ordered what if included on your DP, once the server runs your DP, the check closes. The receipt that prints out does not have a tip line, displays how many credits you have left and room for a signature.
One of 3 things can happen:
1) server doesn't know guest is using DP, guest sticks to plan and wrong copy is given = no tip line
2) Correct bill is given but guest doesn't fill in tip amount. Some guests don't feel the need to even look when the bill is dropped because they figure the food is covered and they can fill in a tip later. They can't because the next bill doesn't have a tip line. The server has to make a choice, either hope the guest is leaving a cash tip, or bring the empty tip line to the guests attention.
3) a manager can override a closed check and charge a tip to your room, this is at the managers discretion and isn't a given.
So! Now that I've confused you lol, I can say either you were given the wrong copy or didn't fill in the tip on the first check and the server couldn't ask for you to tip him.
I'm sorry you didn't have stellar service at your meal, unfortunately this does happen occasionally. Don't feel bad, you tried to remedy the tip issue.