Often this problem can be caused by having too many concurrent connections allowed in your FTP client. If you are using a program other than FileZilla for FTP, please check the settings for an option with a title similar to 'concurrent connections' and reduce it to 3 or less.
Since we recommend the use of FileZilla, here is a guide for how to adjust this setting within that client.
Step 1
Start the FTP program FileZilla, and then go to Edit and then Settings.
Step 2
Under the left sidebar menu on the Settings window, you should see a Transfers option. Click it.
Step 3
Under Concurrent transfers, you have the label, "Maximum simultaneous transfers:", then a text box with a numeric value in it. Make sure that number is 3 or less. If the value is more than 3, you risk being blocked.
Step 4
Hit the button labeled, "OK" ad then exit out of FileZilla and start it up again for the changes to go in effect.
To change the max number of concurrent FTP connections on a per-site basis, you need to File, and then Site Manager. Click on the saved web site you wish to edit, then click on the "Transfer Settings" tab. Change the value in the text box listed there to 3 or less, hit the button labeled, "OK" then disconnect and restart FileZilla for the changes to go in effect.
For those of you having problems with too many concurrent connections via FTP, making these changes should guarantee you do not have the problem again.