There are several types of reasons, and we can ball them up in some categories:

Social & Economic

##Pilot strike We see it when larger aerospace companies or the aerospace control in Europe go to strike nowadays: nothing moves, no passenger gets off the ground. This does stop almost all traffic.

##Bureaucracy The future might be a huge mess of bureaucratic barriers everyone needs to climb over to get his flight. And skipping any might mean to miss not only the next flight, but to go through all of them again.

##Quarantaine Anyone traveling to the stars might need to keep up a very strict quarantine regime, which gets extended for a long time if they even sneeze! Unlucky you, that you had dust on your nose at the wrong moment.

##Overbooked Ever stood at the airport and couldn't get your plane because the plane was overbooked? Maybe the company running the space transport just did the same... and only has a monthly or wider spread flight shedule.

##Religious ban There might be days when the most widespread religions ban interplanetary travel due to some reasons.

#Environmental ##Impact Season Space debris is out there in tons. Some collide with earth all the time. But a spaceship has less protection from passing space rock than the transit station, and even that is not perfectly safe... but better than being out there.

##Damage No matter if it is the ship's coolant leak or something: repairs in space are notoriously tricky and slow in 0g environment. This can delay travel really long, as there might not be replacement ships or parts readily available.

##Transfer Window closed Now, space travel is clearly time-dependent. Due to a pileup of tiny errors like a docking clamp not opening properly, a tiny error in the data input into the autopilot and a hysterical woman during boarding the whole process of getting off station and onto the right facing to intercept earth just took way too long. In extreme cases, half an hour can decide if the fuel in a spacecraft has a chance to intercept a target with corrective burns or not. So the craft has the unheroic solutions of either getting rid of cargo to increase the stored fuels reach... or to return to the station and wait for the next window to open or restock on extra fuel.

There are several types of reasons, and we can ball them up in some categories:

Social & Economic

Pilot strike

We see it when larger aerospace companies or the aerospace control in Europe go to strike nowadays: nothing moves, no passenger gets off the ground. This does stop almost all traffic.

Bureaucracy

The future might be a huge mess of bureaucratic barriers everyone needs to climb over to get his flight. And skipping any might mean to miss not only the next flight, but to go through all of them again.

Quarantaine

Anyone traveling to the stars might need to keep up a very strict quarantine regime, which gets extended for a long time if they even sneeze! Unlucky you, that you had dust on your nose at the wrong moment.

Overbooked

Ever stood at the airport and couldn't get your plane because the plane was overbooked? Maybe the company running the space transport just did the same... and only has a monthly or wider spread flight shedule.

Religious ban

There might be days when the most widespread religions ban interplanetary travel due to some reasons.

Environmental

Impact Season

Space debris is out there in tons. Some collide with earth all the time. But a spaceship has less protection from passing space rock than the transit station, and even that is not perfectly safe... but better than being out there.

Damage

No matter if it is the ship's coolant leak or something: repairs in space are notoriously tricky and slow in 0g environment. This can delay travel really long, as there might not be replacement ships or parts readily available.

Transfer Window closed

Now, space travel is clearly time-dependent. Due to a pileup of tiny errors like a docking clamp not opening properly, a tiny error in the data input into the autopilot and a hysterical woman during boarding the whole process of getting off station and onto the right facing to intercept earth just took way too long. In extreme cases, half an hour can decide if the fuel in a spacecraft has a chance to intercept a target with corrective burns or not. So the craft has the unheroic solutions of either getting rid of cargo to increase the stored fuels reach... or to return to the station and wait for the next window to open or restock on extra fuel.