Are you playing these pieces to full perfection in this amount of time?
If not, then this is exactly why you are having trouble with The Entertainer. Being able to play a piece does not make it finished.
Keep in mind that compared to the Minuet in G Major, The Entertainer is a BIG step up. They are also two extremely difficult types of music. Both the Dance and Minuet require timing over expression, Whereas the Joplin requires both, though interpretation is much more important. Maybe the difference in style accounts for the difficulty?