If each server in the farm has its own copy of the web application files, then it is very likely that there is more than one custom dictionary now and that each one has different words.
To solve this problem you have two choices - move the RadSpell dictionary files to a common folder for all web servers (e.g. a network share), or create a custom dictionary provider and store the dictionary files in a database. If you decide to move the dictionaries, set the DictionaryPath property of the RadSpell control to point to a virtual folder, which is mapped to a network share (e.g. on one of the farm servers). If you decide to write a custom dictionary provider, we have a KB article that will help you - http://www.telerik.com/support/kb/aspnet-ajax/spell/changing-the-custom-dictionary-files-location.aspx
the Telerik team