The log file that CoreFTP (or any other FTP program) generates would help to solve the problem, as this is most likely a problem with your host's server, but with the log file information, you'd be armed with information on how to tell them just how to fix it, or at least what the problem could be.
My log file is located at:
C:\Program Files\CoreFTP\COREFTP.LOG
You need to enable logging in the global configuration of your FTP client, however (in CoreFTP, go to View, Options, then make sure the checkbox next to Log all activity is checked and there's a filename).