Apache .htaccess redirect file - apache

Question

I have written this rule in the .htaccess file:
# Rewrite super sized ciim5 images to large
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ciim5/([0-9]+)/([0-9]+)/super_000000\.jpg$ /ciim5/$1/$2/large_000000.jpg [NC,L,R=301]
What it should do is when the path /ciim5/123/456/super_000000.jpg is requested, it should 301 redirect to the same path, but serve large_000000.jpg instead. Both super_0000000.jpg and large_000000.jpg are actual physical files on that server (effectively we want to hide super sized images from public and redirect them to large)
I've tried this rule on my localhost (Apache/2.4.7 (Ubuntu)) and it worked (rest of the .htaccess file is same as the production one). But when I upload it onto the production server (Apache/2.4.18 (Ubuntu)), it's not doing anything at all. But the previously existing rewrites do work (= mod_rewrite is enabled). I have made sure the .htaccess file is updated on the production site (and it is), and even made a test.html file and open it in the browser just to make sure I'm uploading files in the correct directory.
I'm now sitting here scratching my head, can someone tell me why the above doesn't work on the production server and how to get it to do the redirect?
Edit:
The path /ciim5 is actually setup as an alias to a directory that's not in the web root. So could this be the case of the .htaccess file in the web root not being read? Should I put the .htaccess file in the /ciim5 directory instead?
Edit 2:
Enabling logs shows this:
[Thu Dec 01 10:05:48.444800 2016] [rewrite:trace1] [pid 4572] mod_rewrite.c(476): [client *:33116] * - - [*.org.uk/sid#7f767fb185c8][rid#7f767fa9c0a0/initial] pass through /ciim5/295/491/super_000000.jpg
[Thu Dec 01 10:05:48.445015 2016] [rewrite:trace3] [pid 4572] mod_rewrite.c(476): [client *:33116] * - - [*.org.uk/sid#7f767fb185c8][rid#7f767fa9c0a0/initial] [perdir /static-media/] strip per-dir prefix: /static-media/295/491/super_000000.jpg -> 295/491/super_000000.jpg
[Thu Dec 01 10:05:48.445113 2016] [rewrite:trace3] [pid 4572] mod_rewrite.c(476): [client *:33116] * - - [*.org.uk/sid#7f767fb185c8][rid#7f767fa9c0a0/initial] [perdir /static-media/] applying pattern '^(ciim5/[0-9]+/[0-9]+/)super_000000\\.jpg$' to uri '295/491/super_000000.jpg'
[Thu Dec 01 10:05:48.445204 2016] [rewrite:trace1] [pid 4572] mod_rewrite.c(476): [client *:33116] * - - [*.org.uk/sid#7f767fb185c8][rid#7f767fa9c0a0/initial] [perdir /static-media/] pass through /static-media/295/491/super_000000.jpg
What's this "strip per-dir prefix" thing? Seems like it removes ciim5 or static-media (the actual folder which ciim5 is aliased to). So I should actually remove the ciim5 part from my rewriterule?

Answer 1

So here's what I had to do:
My webroot is in /var/www/*.org.uk. But the media files were in /static-media in the root directory. I had to put an .htaccess file in /static-media and put the following directive:
RewriteRule ^([0-9]+/[0-9]+/)super_000000\.jpg$ /ciim5/$1large_000000.jpg [NC,L,R=301]
Seems like when the request comes to Apache, the strip per-dir prefix (I don't know what this is!) seems to remove /static-media and /ciim5 directory prefixes, so the incoming request (lets say /ciim5/123/456/super_000000.jpg) the .htaccess file in /static-media receives is: /123/456/super_000000.jpg. So now I need to redirect that to the same path but with /ciim5 prefix and to large_000000.jpg. This was done by trial and error really, and I don't fully understand why this works the way this works. So please expand on this answer.