How do you know the items have not arrived?
Have your buyers opened not received cases. As advised, it could just be tracking is slow to update.
Make sure you add any tracking (including the 16 digit delivery code) when you post items and only ever send to the address shown in 'order details' in your sold items.
When you say 'got in touch with them' who do you mean? Royal Mail? Ebay? Your buyers?
Finally, if 'multiple items' you have sent with Royal Mail are not arriving, have a think about what you are doing.
I exclusively use Royal Mail and have had no 'not recieved' cases this year. I had one last year. Royal Mail are probably not to blame.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)