This is similar to "http://sharepoint.stackexchange.com/questions/2057/how-to-write-to-uls-log-in-sharepoint-2010-from-a-net-app". But in my case, logs are NOT showing up in ULS when logging from a web app.
App Pool accounts ARE ALL in the "Performance Log Users" group. Permissions on the LOGS directory seem to be correct, based on looking at other machines where logging is working.
The line of code I'm running in Page_Load of a simple application page, for testing, is:
SPDiagnosticsService.Local.WriteTrace(1234,
new SPDiagnosticsCategory("My Errors", TraceSeverity.Monitorable, EventSeverity.Error),
TraceSeverity.Monitorable, "Exception occured: {0}", new object[] { "NOT!" });
When I write the same line in a console app or linqpad, it logs just fine.
Any ideas of what else I can look at to figure this out?