safari Reader button broken

To my MacBook Air, about a year old and running ML 10.8.2, I dowloaded and installed Safari 6.0.2.

The Reader button in the Safari Address Bar is now constantly grayed out, no matter what pages I load.

I did the same thing on my four-year old iMac, also running ML 10.8.2 and Safari works perfectly.

Does anyone have any idea why this might be?

Both are running the same extensions:

AdBlock

Incognito

ClicktoFlash

1Password

Ghostery

Shortly

Do Not Track Plus

Evernote Web Client

ZoomBySite

Many thanks