How long have you been running the server? Is it a true server, not single player? How many people are playing on it?

I hope this will help you:

SpawnIntervalMin and SpawnIntervalMax
The interval between new Shiny Dino spawns, in seconds. A random value in between the specified intervals is used.

This is copied straight from the admin setting definitions... The default settings for these are 3600.0 and 5400.0. Doing the math, by default the minimum time a shiny will spawn in is 1 hour and the max is an hour and a half. That would mean that if a shiny spawns now, it will take at least 1 hour for the next one to spawn and if you're unlucky will take 1.5 hours (or anywhere in between). Then that same process will repeat... I set the min interval to 300.0 (which is 5 minutes) and the max to 600.0 (which is 10 minutes). You can play with these settings as needed. I would also recommend some crazy number for the max shinies (like 100).

Also the minimum life time for a shiny to be around is 6 hours and the max is 8 hours by default. So if you don't change the spawn in interval then you would at max have only like 8 shiny dinos on a map at a time and that is if every single one spawns at the 1 hour mark.

I would also recommend changing the "NoActiveDuplicateDinos" to False. I think it just goes on to check the next dino if it first checks one that is already on the map, but I'm not entirely sure.

I hope that this will help make your ARK experience better and that you enjoy playing with the shiny dinos!

The interval was indeed the problem, I lowered the values and now everything is working as intented.

Thanks both of you for the answer !