Dependent Drop Down List only partially working

I have made a dependent drop down list which works as expected for my first two columns of options (in 'Schools' sheet) but for some reason does not work when selecting the third option of the initial list ('Cluster' column)

When I tried altering the headings of the two columns that worked (ARM and EPOS) they stopped working, despite being selectable in the inital 'Cluster' column under their altered names.

Googling and playing around has not availed me - any help very much appreciated. I've changed the names of the schools to random strings for confidentiality.

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Thank you!

Check Name manager, names must start with a letter or _, in your name manager you have it with an underscore however in the Schools table it doesnt match to that.

You'll need to either change the name of 2Gether so that it starts alpha or add the _ to the column heading in Schools and reset the Name manager to show the right data