I hate “Invalid bug” label from developers for the bugs reported by me, do you? I think every tester should try to get his/her 100% bugs resolved. This requires bug reporting skill. See my previous post on “How to write a good bug report? Tips and Tricks” to report bugs professionally and without any ambiguity.
The main reason for bug being marked as invalid is “Insufficient troubleshooting” by tester before reporting the bug. In this post I will focus only on troubleshooting to find main cause of the bug. Troubleshooting will help you to decide whether the ambiguity you found in your application under test is really a bug or any test setup mistake.