check your Splunk logs like this:
this will bring up all information about the scheduled searches. You can find more information on other Splunk internal messages if you leave the source from the search.
You can find some pdf related messages in
python.log for example and you can increase logging channels to get more detailed messages back http://docs.splunk.com/Documentation/Splunk/latest/Troubleshooting/Enabledebuglogging
hope that helps ...
please provide more details, like do you want to search for searches that literally failed (the search itself?) or do you want to run a search that searches for events that contain the value