In my experience, most often it tends to be single passengers who fail to show up rather than those travelling together.
Probably because in a group, there will always be someone keeping an eye on the time/screens etc. but a single person can get distracted in the shops/fall asleep and loose track of time.
At LHR there are also a few passengers who decide to just "pop" into London between connecting flights not realising how far it is and how long public transport can take, then missing their flights as a result.
