Let's go back a bit here, do you have any of the following?
1. Objectives to your testing?
2. Target / Load Profile / SLAs etc.?
4. Think Time?
PS. 'Users' is not a description of load. It is not an objective or even a target. It is not a scientific meansurement of traffic. It is not useful. It is not how you should measure or describe your tests. Try getting into the habit of using phrases like 'business transactions per second', 'page views per hour', 'hits', 'end to end journeys' etc. You need this language to even begin to think about doing a good job of reporting a performance issue.
You mention checking memory. Can I assume you mean you checked the server console and there was still free memory available? What about heap utilization? Sounds to me like you have a memory leak in the application, and with such a low volume of users, it may be in the user session data. How large is each?
If all of these are ok, what about database connection pools? Are you freeing up database connections as they are closed?