What this usually means is that the client machine hasn't sent back the version information in a format that ePO can understand. This almost always happens because when it's collecting the properties, the agent on the machine asks VSE, which in turn asks the engine for the information. If, however, then engine is unavailable when asked - because it's busy, or in the process of an update, or something like that - then it can't reply in time and the agent has nothing to pass back to ePO.
You can try the approach in KB60515 to try to mitigate this: it changes the way that the version information is collected. You may find this approach more reliable in your environment.