Mices Problems

I have a cyber cafe with around 20 PCs and few days ago I started experiencing a problem with mices, everyday there is a mouse that stops working and I am oblidged to replace it . Almost all computers throughout the network had the same problem and till now it continues disturbing even new mices I baught , they work for few days and they stop working and I would like to know what is exactly the problem with the computers of my cyber cafe and how to solve the problem because there has been enough time buying new mices and they stop working within few days.
Thanks for your help.