If you are editing your form and it doesn't save but instead sends you back to the Forms Manager or the Main Site Admin screen then you probably have a problem with a server setting.
If the form doesn't save, please see this FAQ.
If the form saves but when it re-opens you only see part of the screen then you probably have an error in the form code. These are sometimes caused by PHP errors that you can see if you temporarily set Site Error Reporting to Maximum in the Global Configuration. You may need to view the page source to see the error if it is in a 'hidden' part of the page.
One occasional cause of these errors is the Header element. There is a bug in some versions which will break the form if a double quote " is used in the Header element code. To avoid this use the Header element only for simple headers and avoid quotes as far as possible, if they are essential use single quotes '.
Note: The Custom Element element in the Advanced elements group will also take HTML and PHP and does not have the same problem.
There was another occcasional bug in CFv4 when you have multiple container elements in your form. The page may fail to save at all or you may get an 'Out of Memory' error message. In this case please update to the current release of CFv4.