1 They have phase delay
Some people have their biological clock tuned in such a way that their bodily rhythms aren't aligned with their routines. This is called phase delay and it might force some people to wake up late. Phase delay can be changed, but it requires a lot of commitment and hard work.
2 They are insensitive to ambient sounds
Ambient sounds or background noise can disrupt the pattern of sleep and force some people to wake up earlier. People who are less sensitive to ambient sounds might have a harder time waking up. See also why you wake up feeling tired.
3 They have problems falling alseep
People who have problems falling asleep might remain awake in bed for a long period of time. This can make it hard for them to wake up on time the next morning. See how to have a good night sleep.
4 They have night owl genes
Studies have found that there are 15 genes linked to being a morning person. This means that a person can be a night owl as a result of having a certain gene combination. See also why you sleep too much.
5 They need more sleep than others
Some people need more sleep than others. If those people don't account for that by sleeping early then they might find it hard to wake up in the morning.
6 They don't get enough sunlight
Sunlight directly affects the human biological clock and helps the person wake up on time. People who don't get enough sunlight might find it harder to wake up since their biological clocks won't be aligned with their routine.
7 They do not sleep early
When a person sleeps late, they might not get enough sleep, and as a result, they might find the process of waking up very unpleasant. If this habit continues then the person might have problems waking up early. See why do we feel angry when we get up.
8 They don't have a fixed waking up routine
People who don't have a fixed routine for waking up might struggle each time they try to wake up. Those who sleep and wake up at the same time are more likely to wake up feeling refreshed.