Mastercard, WHAT is the problem?

What is it with Mastercard? I'm never anywhere near my limit. About half the time it's declined in a bank. Sometimes in a shop they try and process it without a pin and it doesn't work, then someone else comes, asks for my pin and it is OK.

Sometimes at CAB it's declined, but a more savvy staff member comes and does something, with wires or switching machines or something and then it's OK.

Sometimes in shops they roll their eyes as soon as they see it and ask for a VISA CARD.

Sometimes in a shop it's declined on one machine, but approved when they try the machine next to it.

The main thing I want to know is, in the CAB, it may or may not work, and it seems to depend on which staff members are there when I go in, some know how to do it and some don't. It's infuriating though, because the message on the machine is 'declined' (not possible), and if I push the issue and they phone the bank that issued the machine, the Cambodian bank will insist the account is empty (it isn't). If I just knew which machine they need to use or which wires they need to plug in to make it work, it would be so good, as my Mastercard is free to use, and my visa is really expensive.

For the record, the card is a UK issued Clarity card (pounds sterling) by the Halifax bank.

Thanks for any heads up.