I have seen this issue before and I would suggest that you check the version of the add-in against the version of your Excel (go to the about screen and verify if you are using the 64 bit version or 32 bit version) version. Your add-in version must match the bit version of your Excel as well as your ALM version has to be supported, It is possible since each went and got their own add-in that those with this knowledge have installed the correct bit version and those unaware installed the incorrect version.
I have seen that the 64 bit Excel version was being used and the 32 bit version of the Add-in was installed. It installed correctly and says it was active in Excel but it couldn't connect correctly as it was differing bit version. You can only have one Excel add-in installed at a time so I recommend uninstalling all Excel add-ins and then check your Excel version on the about screen and if it says 64 bit then install the X64 version of the add-in and if it says 32 bit or has no bit version listed then install the x86 version of the install and retry to use the add-in. If it doesn't work then register the client and the install the connectivity add-in through the tools menu found on the qcbin landing page for ALM.
Hope this helps,