i bought some from the garden centre along with peanuts and mixed hedgerow-bird seeds. all of the birds including blue-tits completely ignore the "fat-balls". why??

Birds are not daft! they will eat the seed first!
Blue t**ts go for the peanuts first like- wise most of the other birds will eat the seed .
When the first frost comes along then they will go for the Fat Balls or if there are no Peanuts or seed!
They use the Fat Balls as emergency ration's.

Don't use shop bought fat balls, make your own, the birds in my garden love my home made stuff.

Put peanuts and mixed poultry corn in a blender and grind it up, put that in a margarine carton then pour over and mix it up with some melted unsalted lard, put it in the fridge to set then fix the carton to a fence panel or wall and sit back and watch the birds.

we have lots of birds visit our garden...and it is only the starlings who are interested in the fat balls...maybe you dont have visiting starlings? if you do...leave them out anyway...they will find them.