VLOOKUP with CHOOSE - returning #N/A but works using F9 calculation in formula bar

I'm trying to automate a process for populating a spreadsheet (aren't we all?)

To explain the problem, let's equate it to marking exam papers. In this case each paper is marked by 2 assessors. We need to record both scores given, and who gave that score. Other data and comments are needed for other purposes, so I've created an online form which generates an excel download of all submitted scores, feedback, etc. - replacing a previous system where this was all done in a word doc and some poor admin had a full time job copying and pasting!

So, I need to get just the scores from the download into the correct columns of a master spreadsheet.

The master spreadsheet has rows with a unique reference for each paper, and columns with names of assessors (as well as a lot of other data that doesn't come into this).
The download has a row for each feedback entry; each row includes both the paper reference and the assessor name. I set up the online form with the names as a drop down so I can be sure of an exact match.

I've cooked up this formula using VLOOKUP and CHOOSE so that I can essentially generate a unique reference for lookup that is the paper and assessor - as each paper is marked by at least 2 assessors:

=VLOOKUP($J243&R$1,CHOOSE({1,2},'Sheet2'!$A:$A&'Sheet2'!$B:$B,'Sheet2'!$J:$J),2,0)

I repeatedly get an #N/A result BUT - and here's where it's odd - if I calculate with F9 while I'm in the formula bar it calculates it correctly, every time!
Any ideas why that might be?????

I've used a CONCATENATE on the same cell pairings to check that they register as a match, and they do (and with the F9 calculation producing a correct result, that would also confirm this is the case).

I should probably be using an INDEX-MATCH instead but I must admit I'm rusty there and I can't get it to work, when one parameter is in the vertical and one in the horizontal. Any advice on that would be useful also!

Thanks