My thoughts are either the CTEK isnt working properly or the battery is already duff but my money is on the battery being old/knackered so how old is the battery? .
Note that once a battery is abused/worn out not all of them can be "recovered" even by Smart chargers as their capacity to hold current is reduced /lost. It could just be that your car charger gave it a big enough boost, as these often provide high current charges, to "kick start" your dead battery (for a short time).
PS not a good idea to try to regularly boost charge bike batteries as they can overheat/boil dry/ warp plates etc.. long slow trickle charging of a good battery is the better bet.
PS measurement of voltages when on charge and when not may also help the diagnosis but how much effort versus getting a new battery are you prepared to expend?

__________________
ex XT660X Tenere, now XT1200Z Super Tenere