While there are a lot of improvements that could be made, we have been using the WHD approval process without problems ( via web or email ).
Have you checked that the emails are arriving or have notifications set up if the approval email is not formatted correctly?
It is somewhat hidden but you can check both of these in the E-Mail -> Incoming Mail Accounts. Clicking on the configured E-Mail Account opens 3 new tabs.
The notification could tell you if there are problems ( i.e. Vote subject parms missing) with certain approval emails and the E-Mail history should verify that emails are being received.
I don't remember any other settings specific to approvals via email. Hope this may help.