First off, make sure you have selected the 'Email ON' button for your specific Receipt Numbers you have registered on the USCIS website.
Secondly, e-mails are not always generated....meaning, if the LUD changes but there is no change of status on your case, do not expect e-mail.
Thirdly, the e-mail system or the online system for that matter is not 100% fool-proof...I know of folks whose online status does not get updated even after milestones are completed (FingerPrinting notice issued, FP done, RFE response etc)
Best way I think is to keep checking the USCIS status at least once a day....and of course keeping an eye on the service center dates...