It could be a missed CTOT. Calculated Take Off Time. Often called "slots".
When the airways are busy, CTOTs are issued. You have -5 minutes and +10 minutes to depart. Eg, if your CTOT is 09:55z, you can depart at 09:50 to 10:05z. Any later, and good luck. You might get away but usually it's "you're too late, you'll need to refile/get another CTOT". If you're lucky, the new CTOT will be within 5, 10 minutes so that's ok. If not, then you'll probably go back to the gate or remote parking and rethink your fuel plan/duty limits.
An airport slot is often a lot stricter. Some give you ±30 minutes, others (like LEPA), 0 tolerance.. (I hope they've improved that!).
Then there's A-CDM airports. Airport Collaborative Decision Making. Another way to drive us mad. You chat with your handling agent as to when you'll be ready to push/taxi and they sort that out with the airport... It's supposed to be more efficient. If you miss that, then you'll need to chat to the Handling agent, and this assumes your flight plan is still current.
The US has a completely different system....