I marked a lot of spots too ( like 8 ) but i notice that them spawns in any point in particular ( one of these just worked again 1 time after 100 days THATS i call random ) , only happens because that 5% works on that 64x64 square , after that have to wait 1000s to respawn if dont ( because when u enter in each square that 5% maybe not make the respawn but enters in cooldown ) just have to wait again or go to another spot
In the end them are pretty random in their respawn , only go for them at night and thats all , if u missed one maybe u will see that same one at daytime with some luck