Upon reviewing the activity log of a record you've noticed their activity log seems to be missing data.
Usually the activity type is a "Change Data Value" and the details show "Missing history details". If you double click on the activity nothing happens, there is no Activity Details popup window.
This message appears in the activity log when the characters in a field exceed the maximum number of characters allowed in that field. Basically you've hit the character limit for the field. The character limit is 60,500 character. The resolution is to reduce the number of characters in the field. Also evaluate if you need all the information in that field and optimize so that the character limit is not met.