EX4 FTP Maximum User Limit

Dear Friends,

I just bought EX4 for copying data from 15 different location using FTP.

Till the 10 User data is perfectly copying without any issue, but when all 15 location send data to EX4, data copying stop or very very slow even 2 MB file take 15 to 20 minutes to copy.

is there any way i can increase the User limit by using SSH or any other way …!!