Summary has a configurable key called SQLTextSize to limit the amount of characters returned from the Description field for an activity preview which in turn provides increased performance. The reason that some records do not display Summary is that in some rare instances the SQLTextSize truncates an HTML tag within the activity's description field, making Summary incapable of parsing the data correctly. Furthermore, Summary will not display any activities where it is included in the criteria filter results. There are two options to correct this problem:
You will need to change the SQLTextSize to the recommended 4000, but this may cause Summary to break in other records for the same reasons mentioned above.
Pros/Cons: If the customer values performance then they can use the SQLTextSize key, but they may not be able to view Summary in certain respects for all accounts. If the recommended value does not produce the desired results, then increment the value of the SQLTextSize key slowly to get it the best balance of function and performance in your organization.
The second option is remove the SQLTextSize key from Summary.config and eliminate the uncertainty of improperly truncating data.
Pros/Cons: If the customer values the function of Summary on all records then removing the SQLTextSize key will allow this, but performance will decline.
Related KB Articles:
KBA-01151 : Configuring fetch limit in Summary with the 'SqlTextSize' key