In the past few weeks I have been sporadically receiving update alerts for new chapters to stories I'm following. Sometimes I receive the alert immediately, other times it takes several hours, and in really bad cases I've received no alert at all (I just happened to visit Dokuga and catch the update on the main page).

I use gmail and regularly check my spam folder. My alerts used to work perfectly, so I'm curious to know if any changes have been made internally which could explain the discrepancy?

This problem has actually been ongoing for a while and is affecting everyone. I believe it is on Miss K's list of things to do so it will be fixed as some point. So don't worry. It's not just you. ^_^