Why? Where did my data go? - Read On...
The disappears because its TTL has expired. TTL is the Time To Live. The TTL is used to indicate how long the data from the UA is available for evaluation. The TTL value not affects what data is available for situations, but it directly affects what you see in the Universal Agent workspaces in the TEP.
So here is why this happens:
If you defined an attribute in a UA metal file as E or Event type data in the metafile NAME statement, you do not need to specify a monitoring interval for situations using attributes from that group.
If the attribute group is S, P, or K - (sampled, polled or keyed) data types, you need to take the time-to-live (TTL) value into consideration when you set the monitoring interval.
How do I set the TTL? Good question - in your //NAME definition you define it as the third parameter.
//NAME myattributegroupname K 1200
This sets this to Keyed data with a TTL of 1200 seconds. I recommed at least setting the TTL for 3 times the situation interval, so if your situations are polling every 2 minutes - set the TTL to at least 6 minutes (360 seconds) to ensure you always have data available for the situations.