This isn't a problem with your connection or ClassReach, but your Gradebook Settings.
If you are a Teacher or an Admin and entering grades in the Gradebook using a letter or another alternative mark, other than points, that letter/mark must be listed in the Grade Mappings section of the Gradebook Settings. If it's not listed, then the entered letter will subsequently disappear after it's entered.
Using the Gradebook Settings below, if a teacher enters a U as a grade for a student assignment it will be rejected and disappear after it's been entered. This is because the U is not listed as an option under Grade Mappings. The Grading Scale is only used to assign a letter grade for the overall average grade. These two, Grading Scale and Grade Mappings, are not directly related. See below.
If a letter/mark disappears after entering it is being rejected as an eligible mark.
See the following article for an explanation of Grade Mapping, Grading Scale, etc.