Lync 2010 and Lync 2013 blinking icon in taskbar not working properly

     Hi,

     We're experiencing a strange behaviour. Lync IM notifications are supposed to work this way below:

     1) Notification bubble on the right-bottom corner

     2) After a few seconds, a blinking icon in the task bar

     3) After a few seconds, a coloured icon remains in the task bar

     Some of our users, both Lync 2010 and Lync 2013, go 1) and 2) but then icon won't remain in the task bar, so they don't realize they have a conversation waiting (Windows XP and Windows 7).

     If they open the icon, the conversation is there and coloured, and they can answer.

     In my computer, it has also happened but most of the time it works fine (testing it, maybe it works 15 times and fails 1). Nevertheless, a lot of people is suffering from this strange behaviour.

     We have looked into the causes but haven't found anything.

     Any clue?

     Thanks!