Hello there @Alexpyd You have been quite unlucky here. The odds of Mirage Island being visible increase with each attempt. On your first attempt, the odds of the island appearing are 1/16, but each day you return and speak with Old Man Jerry again, you will have a higher chance to see Mirage Island (2nd attempt 1/15, 3rd attempt 1/14, and so on, until the 16th attempt which will be 1/1 or 100%). You can find out everything you need to know about Mirage Island here.
If you have any other questions related to this topic, please let me know.
Houndstone