Could be a number of things...
It's more than likely it was just lost at the local ATC unit. I've called for start many times and as far as my own Ops department is concerned our FPL has been acknowledged and filed by BRU but it doesn't appear in the local tower!
Or the crew's paperwork was missing...
I would also imagine (someone in LTN can correct me) that easyJet Ops use RPL's, Repetitive Flight Plans (Same route, time and date) and software today is advanced enough to avoid what Student88 is suggesting..