I got the same issue (not charging) with the same symptoms (red light for ~5 seconds).
problem solved - I changed to a new, "higher quality" usb cable.
(the old cable was 3 years old and has had a hard life)
Looks like the Purity Pro is fussy about it power source.
it stops charging after 5 seconds if it thinks something is wrong.
I changed to different chargers and USB cords but nothing on my end.
Do you mind sharing what brand of charger and cord you're using?
Thanks,