OK,

Then lets work through the options -

Firstly, I forgot to mention, the amber/green indicator has nothing to do with anything to do with your problem. It can safely be ignored.

Have you set a value for the postcode in Shipping and Packaging?

Have you edited/modified the Package Tare in Shipping and Packaging?

What is the product weight of the item whose cost is being estimated?

What is the current/default country shown in the shipping estimator drop-down when you get the 'rate cannot be determined' message?

The fact that you are getting that message means that the weight and value of the shipment should be OK, but it doesn't hurt to check.
Other than that, it simply loops through the weight/costs table looking for the price for the weight of your item, and the message suggests that it was not able to find one.

If it's only 1 module doing this then check below, otherwise it is a setup/configuration problem.

This could be due to an error during installation, or an error in the price/weight table.
Check in the Admin-Modules-Shipping for the price/weight table in the module giving you the problem. e.g. in RMFirstLargeLetter, it should look something like this - .1:0.9,.25:1.2,.5:1.6,.75:2.3
It is simply a list of weight and prices separated by a colon, with the groups separated by a comma - O.1kg costs 90p etc etc
Check that the colons or commas are not missing or mistyped in the misbehaving module.

Hope that helps,

Chuck