I sell WordPress themes where I include a Twitter widget. In the widget I use a WordPress transient that refreshes every 3 hours to cache the Tweets. I've got thousands of people using this and it seems to work for just about everyone. Also, I've done little tests on my end to ensure I know the cache is working properly with how I have everything setup.
However, I've run into maybe 2 or 3 buyers where it's apparent that the cache is not working at all on their servers.
Note: I can tell because they keep getting timeout issues as they're hitting the Twitter API more than the allowed 150 times in an hour.
So, are there any reasons why someone's specific web server could have problems with WordPress transients? Or maybe something specific to someone's WordPress configuration? Is there anything else I could be overlooking?