There shouldn't be any reason for entries to suddenly disappear after a change is made unless there's a Search Option involved. To ensure no Search options are active, go up to the white Search field at the top of the window, click the magnifying glass symbol inside it, and select 'Clear all Search Options'.
Also, since you're using 2.5.7, which is an older version, you'll want to update to get all the latest bug fixes. To get the latest version, visit our site at
http://www.splasm.com/checkbookpro/update.html and follow the steps below 'Get the update'.