Its a little unfair to said its "their fault" that a slot was missed. There are a huge amount of variables, stakeholders and parts of the supply chain that go in to getting the craft back in the sky. It only takes one to be delayed and its game over for the original slot. This can also include weather, vectoring, runway changes, single runway ops, extended taxi in, slow boarding, missing pax, missing bags, too many bags, unable to fuel due to a CB in the vicinity, an aircraft pushing back behind you and blocking in, a so called 'ramp check' by the local incompetent Authority. When the jet is ready and there is not a slot available then it really is air traffic control restrictions causing the problems. I won't name any country (its not the UK) but priority will always be given to their local airlines too so us jolly foreigners are at the back of the queue.
Now we have almost weekly strikes by some countries. When they are not striking they are working to rule and flows must be reduced. Even if you are ready early then you will often have a slot given to you regardless.
Some places are now busier than ever. Barcelona had arrival restrictions yesterday due to the wind as they could not operate parallel runways for a few hours. PMI is busy and being #14 for arrival and #12 for departure is standard. PMI ground cannot cope and you cannot get your ready message. I observed a Helvetic crew try to call ready. They could not get a word in and when they could they were told they were too late and that they had missed their slot whereas in fact they were ready on time just the ground controller either ignored or did not hear their request. Thankfully the crew stood their ground and away they went.
Its very easy to take a simplistic view (particularly when people are looking forward to their EU261 money) but in reality the operation is vastly more complex than passengers could ever imagine.