Why am I missing Metric data?

I Recently got an alert that a DB was approaching "process_usage" limit.  Thinking I had seen it before I searched past email and found that the alert came out each week at the same time.  I wanted to look past history of this metric for clues, but there was no data to retrieve either through "All Metrics" in the console nor by querying the repository directly.  Looking further (both through the console querying the repository) and I find this data for some DBs but not others.  All DBs use the same monitoring template, and looking at monitoring settings I see no difference