A few month ago, I noticed that from my laptop (Windows 7 professional 64bit + IE 9), sometimes I cannot see the "Welcome menu" ("Personal Action Menu") from SharePoint web page.
Other users didn't get this problem.
I am the SharePoint Farm Administrator and windows desktop Local Administrator, and I can see the "welcome menu" from other computers or other web browsers.
No one changed the master page.
Then, what caused this issue?
After quite a while struggling, I finally found the cause: the default IE "zoom level" was changed before.
How to fix it? Change it again. :-)