How to start 
Summarizing the issue:
I have followed the documentation for installing the snap-store-proxy here for my company to reduce the internet traffic in a Ubuntu machine 20.4, and whether in the beginning it does the job (I can see the images downloaded and my hosts can download it locally quite quickly), after one hour, the packages remains in the server (at /var/snap/snap-store-proxy/common/nginx/cache) but are being ignored and re-downloaded again keeping same name, and same download folder in the cache.
In addition to that, the approval is pending since a week already with no information how to proceed to request such approval.
Hosts are able to download the packages from the proxy, but after one hour, I notice that the proxy ignores the already downloaded files, and it tries to download it again (with same name, in the same folder).
You can see an example here, I have install early this morning 11 packages, and this afternoon I re-run the intallation for the same packages, and it tried to download them again from internet instead of using the one locally downloaded.
total 4.3G
604M -rw------- 1 root root 604M Dec 3 14:49 07955bc6b832f31ffa35681e8c044d52
98M -rw------- 1 root root 98M Dec 3 09:02 2d905c6ab1151f8c85f64b4ad22644d7
767M -rw------- 1 root root 767M Dec 3 14:50 427756a08ccb55a79d29b618b9a66702
168M -rw------- 1 root root 168M Dec 3 09:00 5b9f8361c339b78dc950790342d85031
278M -rw------- 1 root root 278M Dec 3 09:00 5f23ddbaa62b93b59ad10445655b88f8
670M -rw------- 1 root root 670M Dec 3 14:50 6e008101a15241076bded13d0684bd66
223M -rw------- 1 root root 223M Dec 3 09:00 8950629852dab294227043c281762258
818M -rw------- 1 root root 818M Dec 3 14:50 902c0b41d1c150ca1a57fe03131fedc6
254M -rw------- 1 root root 254M Dec 3 08:59 cb309ef1ee00ae8152d2bb8e16179455
82M -rw------- 1 root root 82M Dec 3 09:02 cc6a85bfe7a313fedb5bac0cca209394
68M -rw------- 1 root root 68M Dec 3 08:59 f0bfdeefd89e208745fd156a53db7006
296M -rw------- 1 root root 296M Dec 3 15:49 fd537f5f80039016bfb8f2479a774a28
I measured that after an hour, it tried to download it again.
I tried increasing the cache time with 438000 minutes, but still the same issue and after one hour, the content is being re-downloaded again from internet:
proxy_cache_path /var/snap/snap-store-proxy/common/nginx/cache keys_zone=snaps:1m inactive=438000m max_size=8192m;