Problems when adding an item which has a weight

[confused_aswell]

Hi

I have zones set up and also free shipping for when they spend over £15. But when I add an item to the basket which has a weight to it the zones tell me that "The shipping rate cannot be determined at this time"

I can't seem to get round this, is there a way of getting them working together so when someone buys a heavy item in addition to what they have bought that it doesn't muck up the zones?

Any help please.

Thanks

Phil

PS Version 1.38a