Several possible causes.
1. Text box size
Make sure the text box is large enough in the report, or has its CanGrow
property set to Yes and the CanGrow of the (Detail?) section is also set to
Yes.
2. Aggregation
If the report is based on a query, check that the query is not doing any
aggregation on the memo field. For example, the query must not contain
DISTINCT. If the query as a GROUP BY clause, change it to First instead of
Group By under the memo field.
3. Formatting
Make sure there is nothing in the Format property of the memo field in the
table, and nothing in the Format property of the text box on the report.
4. Exporting
If the problem only shows up when you export the report to RTF (not when you
view or print the report in Access), see:
http://support.microsoft.com/?id=288877