Hey! Thanks for the comment!
The Lisa ending should only appear if you choose "I want to be with Lisa forever." If you choose "Lisa will always be with me," the game will continue the same dialogue for a while-- and then you should enter summer C015 as the story goes.
If you still encounter any problems, please do not hesitate to send an email to cherishfan0320@gmail.com so that I can provide an unlocked version for you to play. ^^