The Instant Scan depends on the inotify watches system, provided by the Linux Kernel.
Every Linux installation has a specific amount of inotify watches set by default.
If you want to know how many inotify watches your system has, execute the next command:
You will need one inotify watch per watched directory. Thus, the more directories on the server, the greater the amount of things that will be needed.
You can increase the number of inotify watches performing the next command:
Increase the number 999999 as your needings.