Well, without being able to see the code i can just guess.
But if it only works for one Ai and all others are doing nothing or seem to just hang, my first guess would be that you accidentally used a static variable (which is shared among all instances) somewhere instead of a normal member variable (each instance of that class having its own).
For example, you might have a normal member variable “bool BusyDoingSomething” which gets set to true when the AI is busy doing a task (like walking or following a target) and false when the Ai should decide what to do next.
If it is a non-static variable (aka no static keyword : “bool BusyDoingSomething”) then it should work fine, since every Ai instance will have its own.BusyDoingSomething-check.
If however you have it as a static variable (aka “static bool BusyDoingSomething”), as soon as one AI sets it to true, all other Ais will also see it as true, so only the first one being able to set it to true will be able to do anything. Once it gets set to false only the first Ai setting it to true will then be able to do the next thing.