Nginx caching not working for crawlers

I have been using Nginx caching for years but recently noticed an issue which I suspect has been happening for years.

If I access a page that's never been visited before I of course get a cache miss, refresh the page and now its a cache hit.

However I noticed when crawlers such as Bingbot or Googlebot crawl the site, the pages they crawl never get cached, they always remain cache misses until I manually visit them.

Is there a setting somewhere in Nginx that prevents bot traffic from caching pages?

FYI I am not treating bots any differently to users in my Nginx config.

Here's a sample of my config.

    proxy_socket_keepalive on;
    proxy_redirect off;
    proxy_cache Google-Cache;
    proxy_cache_revalidate on;
    proxy_ignore_headers  Expires Cache-Control;
    proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504;   
    proxy_cache_background_update on; 
    proxy_cache_lock on;


   proxy_cache_valid 200 206 304 306 301 302 404 410 300s;