Why Would Cache work differently on a subdomain?

I have this line of code on a domain.
Cache.Insert("storeTweets", Source, Nothing, DateTime.Now.AddMinutes(15), Cache.NoSlidingExpiration)
It handles caching of our store tweets and works perfect.
However the exact same line of code on a subdomain of the same domain does not seem to be updating after 15 minutes, it does not seem to be updating at all.
Any guesses as to why a domain would handle cache differently to a subdomain?