Hi, thank you for the answer. I began watching this issue closely and IT LOOKS LIKE the update link is not present when the newly added version of a plugin is just a few minutes old.
As I learnt yesterday, when a developer submit an update of any plugin to WordPress, there are about 20-30 minutes of delay until it is finally published and made available for download.
This week I was checking closely the updates of BackWPup, which upgraded recently with a lot of bugs, and the users was angry and desperated for an urgent update.
May this issue be due to the above explanation? I will check that.