Good to know about that tweak, but I don't have that installed. Wonder if it could be 'No slow animations'. or 'Speed Intensifer'.
Check your list of installed tweaks, some tweaks automatically install Activator.
I have activator installed and the double tap not working is intermittent. Try killing all apps and resetting the phone.
"Assign all your fingerprints to "do nothing" but leave the single press, short hold, and double press alone."
This drastically improves reachability for me (6 and 6+). With activator my double tap has always been iffy regardless of iOS version until I did this. No other sensor tweaks installed.
Good to know about that tweak, but I don't have that installed. Wonder if it could be 'No slow animations'. or 'Speed Intensifer'.