Did you just upgrade to excel 2013 and realized that your excel addin can be used only in the first instance of the excel. This may be because Microsoft has doesn't support MDI any more in Excel 2013.
You can fix the code and redeploy the add in that will take care of the problems, you can find what is changed in the article below.
If you are busy and don't have time you can try the following work around.
Try running all the instances of excel using "excel /X" which will open all the instances of excel in its own process.