You would probably find it very beneficial to migrate up to version 4.x from 3.4.6 as well.
There are various reasons why a search may not return, but typically they can stem from the following:
If you have run wild card searches that span all time ranges for all indexes (including _internal) and receive no results, then I suspect you have a problem with the indexes. For this situation you should log a case with Splunk support.