Why does Find my iPhone says iPad is offline when it is not?

Hello,

Our company owns 20 iPads currently and we use the same Apple account for all of them. They all have Find My iPhone installed as well. Many of them however show as offline when they shouldn't. They are all connected to WiFi at all times, they are all on, the dates are correct, they are all running IOS9, the location services is turned on, and we are located in the US. What is the problem and how do I fix the issue?

Megan Brown