Shipping carriers option not transfering - based on weight

[roastchicken]

Hi

I have 3 carriers

1. royal mail (0 – 2kg) at £2.00
2. royal mail (2 – 4kg) at £2.50
3. Parcelforce (0-10000kg) at £10

When the overall weight exceeds 4kg, I would expect the Cart to drop into option 3 (Parcelforce) but it remains with Royal mail ( £2.50) ? I then set the ‘Out of range behaviour’ to disable but now when I exceed 4kg, delivery goes to £0. So the cart on the home page does not show correct total.
But when I then goto the checkout, the cart then re-adjusts for Parcelforce and now shows the correct amount.
All carriers are are shipping to the UK from the UK i.e. in the europe zone.

What am I doing wrong here ?