Cannot use Conditional Formatting in Workbook

The problem is simple enough to state. When I select a range of cells (as numeric values or indeed as values calculated from a formula), I cannot conditionally format them. What I want to do is to colorize numbers highest (red) to lowest (blue). I have done this before a million times by using:

1. On Home tab Conditional Formatting > New Rule.
2. In the New Formatting Rule dialog box, "Format all cells based on their value".
3. Set color scale.
4. Ok.

Simple. But this just does nothing now. Is there a setting I am missing or may have accidentally changed? I have tried Shift + F9 to recalculate the range - NOTHING.

I am a broken man.

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