Mistake #1. Chasing the dog
If anything; run the opposite way of the dog.
I would suggest using a "reward system" (food) every time you call the dog to you. In the house; out of the house....every time for about a month.
Condition the dog that coming to you means good things. Food. NEVER a correction. NEVER.
I sure wouldn't allow the dog to be in an "open" area off lead for a long time. Dogs need to be proficient on lead before you can even consider trying it off lead.
It takes time, conditioning and training.
I'd start all over. Forget the whistle (a negative response has already formed into habit with it) change commands from whatever you were using to another "come" command.
Give the dog a clean slate to work from and build a good recall foundation with food. ANYTIME you call the dog to you...have that food reward ready and waiting.
Good luck and happy recall
