Whenever a cue is triggered which contains Colour, Beam, Shape or Position parameter values, these values are sent to the fixtures. This cue has become the latest instruction for the fixtures, and hence this behaviour is called Latest Takes Precedence (LTP).
Intensity however behaves a little differently. Intensity will by default mix between playbacks (and UDKs) with Highest Takes Precedence behaviour (HTP), meaning whichever playback has the highest intensity percentage for that light, is the one that has control.
However, if a fixture's intensity is being controlled manually (and therefore not from any cues or UDKs), this has the highest priority on the console. As manual values have the highest priority, outputting cues will not be able to take over control of a fixture's intensity. Manual values are indicated in red in the Output Window.
This is where the Clear button comes in. Clear allows you to remove all manual values, sending them either back to defaults, or allowing any cues to then take control of the fixtures.
Pressing Clear once clears the selection, and a second tap of Clear removes the manual values.
Therefore it is vital before playing back cues to check if the Clear button is lit. If it is, give it a double tap, to ensure your cues will have control over all lights.