Why can't Siri locate contact addresses for map directions?

Question

Having an issue with iphone 4s using Siri for locating addresses saved within contacts for map directions.
The problem is only evident on about half of the contacts so far..  the message i get is 'I couldn't find an address for .." even though the contact I asked for clearly has their address saved.  I've doubled checked I've used the right address fields and seem fine.
I'm unable to distinguine the problem between the contacts where Siri works as it should and locate the address, and the ones it can't - including my home address. 
Very frustating when I want directions in the car for driving and using hands free.
Any ideas guys?!

Answer 1

It seems to be a bug in the software. Other people have said it may have so something to do with siri not understanding UK postcodes properly.  I don't really know but I seem to have found a solution, although cumbersome.
-Load up your contact from phonebook app
-Tap the address (apple maps should go straight to it)
-Double tap home and switch back to phonebook app
-Delete the address
-Diuble tap back into maps (the pin point should still be there)
-Press the little blue arrow on the pin
-Add to contacts > Existing contact
-Select contact
(Now this is the important bit)
-Tap edit then tap the right hand column off the address section in order to set it as home, work, etc.
-Done
Phew!!
It's long winded I know, but now I can just tell siri to get directions to so and so's home safely while driving without fiddling around tapping buttons.
Sort it out Apple. It should 'Just Work' outside of the US too you know.