array formula doesn't work

Hi guys,

Can you tell me why it is not working as i wanted to?

Task was to check difference for Criteria1 if found in a range for Criteria2 if it is true. Example attached.

I am not sure what you are asking could you explain what you want to do in words.

Is if the difference between green and the next green
=G2-INDEX(G3:G6,MATCH(F2,F3:F6,0))

=iferror(G2-INDEX(G3:G6,MATCH(F2,F3:F6,0)),"")

exactly this, thank you

i just wonder why array formula is sometimes working and sometimes not..