As ATC in an all-singing, all-dancing, electronic trickery tower, we cannot enter a DLA message for an aircraft once it is taxying.
It’s a pain tbh. If it becomes clear during taxy that, for some unforeseen reason, the aircraft cannot now make the CTOT window then it requires telephone co-ordination. If that can’t be achieved (workload at either end of the phone) then it’s either remote hold, if possible (fuel/airport infrastructure), while ATC tries to get co-ordination for a new CTOT or it’s a return to stand and send DLA electronically (in which case aircraft will go to the back of the queue) - either way it’s additional workload.
Rest assured that ATC want to get you away in CTOT window - and will try to do that as best they can - and will try to get some form of extension where possible under those circumstances - it’s a whole lot better for everyone concerned (...but please, if there is a foreseen reason why you cannot be ready for departure at the holding point at CTOT - not CTOT +9mins - file an amended EOBT as early as possible...give us all a fighting chance!).