Cannot update data on multiple instance of same Exe file(New)

I am create a Desktop app like master slave model by single Exe.The exe file act as a master as well as Slave.When one instance run as Master it get data from Sql Server and write to File using Timer(). When it act as a slave Mode It read the data from file which the date wrote by the Master instance using FileSystemWatcher() when the file is modified Slave is access that file and update.

When open more then 3 Instance as a slave in same Machine one of the slave stop updating data.if close any one slave instance then the 3 rd slave instance start the update.Please tell what my problem.

Thanks for your suggestion.