EndNote uses Word's hidden field codes when formatting citations, and inserts all the reference information for all the grouped citations in these codes. There is a limit to the amount of text that Microsoft allows in a single set of field codes, which may be hit if you are inserting 20 or more citations for references that have extensive information in them (such as large abstracts).

To avoid this issue, instead of inserting a large group of citations all at once, insert a small group of references, then a space, and then insert another small group of references. This way, you won't have a single group of formatted citations that would exceed Word's field code limitations.