Unable to cache queries sometimes

In my website, there is an expensive query which takes approx 6 secs. After I activate this plugin, this expensive query is cached well(checked using query monitor plugin). But after a few minutes, it seems like that expensive query isn’t getting cached anymore. But if I flush the object cache, this problem seems to be resolved for some times. But again, after some time, this issue arises. What could be the problem you think? We are using cloud server(512MB RAM) and installed Redis manually using Linux commands.