Outlook 2007 Add-In Does Not Load on Outlook 2013

Question

Hi all,

We have built a VTSO office add-in, for outlook, word, excel and power point, using Visual Studio 2008 (C#, VSTO 2005 SE) targeting office 2007 as the minimum version. 
It loads and work fine on 2007 and 2010 version of outlook, word, excel and power point. 
It is also loading up on 2013 version of word, excel and power point but do not load on outlook 2013.
We have already tried with setting "VSTO_SUPPRESSDISPLAYALERTS" variable but it is not displaying any error, also we have checked it in disabled item list in File => Options => Add-Ins but it not displayed there either.

Any help will be appreciated.

Thanks in advance,