There can be many, many reasons why flights are delayed.
When you have for example a small fleet with not every day flights such as the charters, you can catch up in the down time, but also when an aircraft goes tech you can be seriously delayed then (as FlyGlobespan found this year)
Giant airlines such as BA can have spare aircraft sitting around to take up the slack yet they willonly do so in extreme circumstances. Flying out a crew and and aircraft empty costs thousands of pounds and can be more than the profit on a route for a week. BA have the buying power also to get first slots if an airfield has delays on take off/landings (as Heathrow and Gatwick do all the time)
Virgin are in between. they don't have aircraft and crew sitting around, they make profit by keeping aircraft in the air and passengered up.
Now so many things can effect the take off.
For example, on my last two Virgin flights, the following occurred
1) A passenger on the incoming flight fell seriously ill and there was due to their condition a lengthy delay in getting her off the aircraft, which put the cleaning crew back, then of course t5he slot was missed, the outbound pilot has to then negotitate a new take off and route as when you take off outside your allotted time, it effects the whole route and your effect on other aircrafts doing the same route
2) on the way back, frirst of all, the cleaning crew being down on numbers, i.e. no-shows, were late cleaning the aircraft, then a passenger who had an accident whilst on holiday and was on a strtcher had to be carefully loaded onto the ship, then when leaving the gate, one of the engines would not fire, being a jammed starter.
No airline can build in these delays, its not like many things that work on clockwork, too many variables can happen. Can you really get in your car, leave dead on time and then arrive on time very often? Always things on your journey effect these times.
You will arrive when you arrive. Don't get worked up about it, you are on holiday, relax.
