If the link type is an Inner Join, then it will only show parts that have activity. You can change the link type to be an outer join so that it shows all records from part number even if there are no matching activity records.

Do you have any Select criteria on the report?  For instance, restricting your activity to a certain date range?  If so, that will still mean that parts with no activity aren't being returned, because a result of "no activity" is not in that date range.

To correct for this, you can do two things.  If you have knowledge of SQL, you can do a conditional join.  You would either do it using the Add Command function when selecting your data source in Crystal, or build the query in your database and use it for the source.

The other option is to modify your Select criteria to allow for null values.  You'll have to click on Show Formula and edit it there.  Look for where it references the activity table.  Put an open paren "(" in front of that expression.  After the expression, put "OR IsNull({ActivityList.MyField}) )"
Repeat for every field from the ActivityList referenced in the Select criteria.