I have a 2010 site with 3 web part pages containing a few list web parts. Occasionally a user will select the checkbox to edit an item but no ribbon appears. These users have permissions to edit these lists (and can make changes if they go to the list itself). Other users in the same permission groups have the ribbon.
I've set the navigation setting "Make Show Ribbon and Hide Ribbon commands available" to No. When I changed this setting the first time a user had no ribbon, it fixed it for her. However, I've had 2 users since then experience the same problem. The users have the ribbon on other sites, and a different user with the same permissions logged into the same computer and has the ribbon, so I don't think it's an issue with the client machine.