Hi Fraser,
INDEX/MATCH works better when the data is in a contiguous block where the row and column indices are in one row and one column respectively. I re-arranged your data, on sheet "Data". You can see the indices highlighted in blue. Note that the dates on row 6 are all on the same row compared to before when you had them on three separate rows. This makes it possible for MATCH to look in one place to find what it needs.
Note - the data in pink is now redundant. I didn't delete those columns because I didn't know what you use the header data for.
I put your summary list on sheet "Summary" and changed one date (in yellow) where you seemed to be looking for Nov instead of Oct.
If you find you don't need some of your header data, you might also want to start using dynamic named ranges for this lookup. It will save you the requirement to change your formula ranges when you add new data.
Cheers,
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