Alright, so every few days there is a chance this Wailord will respawn. Basically it is like dice being rolled, you might not land on the number you were hoping for, or in this case the chance of the Wailord respawning like you were hoping for. It is not currently known how many days are in between the chance for it to come back, and you have to play the game for a day to count. It takes people anywhere from a few days to a few months from what I've seen. I hope it respawns for you soon!

Edit: here are some sources to prove this:

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