Do you live in a flat? If so, is there a central area where post is put (eg pigeonholes), or does post go directly through your door? If the former, it could be that stuff is going astray in the property.
I used to run a couple of subscription magazines and we found that many non-arrivals were down to things going missing at the address. UK Royal Mail is very reliable indeed in my experience -- especially compared to Southern European countries and the USA (we had some absolute nightmares getting stuff delivered to some countries!).
Maybe try getting them to send it to your work address for a while?