No problem that you are a "driver" and not a "mechanic": But since you are the one encountering it, we need your help :-)
"Sometimes" is a very general statement. To reproduce something we don't encounter ourselves we need something exact to actually reproduce it. So again if you haven't review the ZP debug and the server PHP error logs. That is the first thing you should do when encountering issues.
Quote:Another separate issue is that I have selected 'Notify me of follow-up posts via email' option but I didn't receive an email (yes, I've checked my junk mail etc.)
If that is an option of the ZPBase theme please note that the theme is not officially supported and sadly its developer is "missing in action" currently.
The only idea would be that you haven't enabled an email handler plugin in Zenphoto itself. Without that it will/can't send any mails.