Thanks to a kind PM I think I have now confirmed in all probability what went wrong.
As those who have taken COMPASS will know the multi tasking exercise involves entering data that appears on the bottom right of the screen into an 'autopilot' panel on the left side of the screen. While doing this random red squares appear in the top left and top right of the screen and depending on exactly where they appear in their respective boxes you have to press corresponding keys on the number pad.
My understanding of the test was that if for example a red square appeared in the top left corner of its box one presses the 7 key on the number pad to acknowledge it because out of the nine keys 7 corresponds to the location of the square inside the box, 1 for bottom left, 9 for top right, so on and so fourth.
What I think I did was hit 'number lock' in response to a top left red square instead of hitting 7. We were warned at the start of the test to ensure that number lock was off because this would mess up certain aspects of the tests. I can only assume that having it 'number lock' early on this stopped the system from registering all my further key entries on the number pad.