Do you have a screen protector/glass on?
I have already started a thread on this issue and this problem is not just with double tap to wake, it's much more than that.
Link to main thread: https://community.phones.nokia.com/support/discussions/topics/7000023193
Double tap is working for me. The screen lights up and shows time and notifications. (that there is a mail from this forum in this case) (TA1043/8.1/no screen protection)