Why hasn't the UK Postal Service delivered two envelopes I have sent to a friend?

The address is right and I put the correct number of stamps on, plus I have sent previous envelopes to this particularly address with no problems. The strange thing is one of the envelopes made it while the others didn't, and I sent them on the same day. What could the problem be?Why hasn't the UK Postal Service delivered two envelopes I have sent to a friend?
Some times things just get lost. It's so annoying. When you send something that is important it's best to send it by recorded delivery. This way it's less likely to be lost and you can also check it got delivered.Why hasn't the UK Postal Service delivered two envelopes I have sent to a friend?
It's been lost in the post. I might turn up eventually but there's no way to know. These things happen.Why hasn't the UK Postal Service delivered two envelopes I have sent to a friend?
The Post Office loses millions of letters and packets every year
I do not know. Who would ? Perhaps the postman is keeping them for a day when he is not busy. Kindest regards