Why does look-up not work on every webpage in Chrome?
This bug is known to the Chrome developers and has been filed under...
Issue 90461: Dictionary keyboard shortcut doesn't work in Lion
Basically the problems are timing issues for some websites which take too long to respond to information needed to show the dictionary.
This is what @resek@chromium.org says:

There's likely not much that we can do here. We have to block the UI
thread to implement this feature, which is bad and so that's done on a
timeout. If the web page takes too long to respond for the information
we need to display the popup, we give up and unblock the UI. If we
didn't do this, Chrome would look like it's hung, waiting on the web
page to respond.
We collect timing information about this, and most pages succeed well
below the timeout, but there's also some class of pages that do not
respond in time (more than 1500ms, which is what starts to be
perceived as sluggish/hung).