Couple of causes for this.
1. Aggregation
If the report is based on a query that performs any kind of aggregation
(e.g. the query has a DISTINCT predicate or a GROUP BY clause), the memo
will truncate. In the case of the Totals query, use First instead of Group
By under the memo field. Because Access does not have to group by the memo,
it is free to return the entire field. Note that the name of the field will
change to something like FirstOfMyMemo, so you have to fix this up in the
report.
2. Formatting
If there is anything in the Format property of the field in the table, or
the Format property of the text box on the report, Access will truncate it
after 255 characters also.