In the future please provide as much info on your particular AI setup as possible. But you’re lucky I’ve stumbled on this, because I know exactly what’s going on 
You’re using our Detour Crowds wrapper, and there’s a configurable limit to the number of active agents. You can guess what’s the default value 
Unfortunately this property is not exposed to any of the editor tools, but you can set it via the ini file. Just add the following text to your DefaultEngine.ini:
[/Script/AIModule.CrowdManager]
MaxAgents=X
where X is whatever you think your limit is.
Cheers,
–mieszko