Sometimes one can tell by examining the events what changes need to be made. In that case, go to Settings->Source types, select the appropriate sourcetype, and made the needed changes.
Other times it's not so easy. One approach is to export some problematic events (raw) to a file. Get them from the original source, if possible. Then use the Add Data wizard to upload the file and experiment with props settings until you find the combination that works. You then can update the app as above or put the settings into an app and upload it.
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If this reply helps you, Karma would be appreciated.
