I am a Ph.D. student in the third year, and I am working on my dissertation with two supervisors since my thesis is interdisciplinary. I recently noticed that my 2nd supervisor has not responded to my last 4 emails although I see he is active on his email. The answer to two of my emails could be even a simple affirmation! The first time I sent him part of my argument and asked his opinion, which he didn't respond. The two next times I sent an email to both my supervisors, which he didn't respond either. The fourth time, I sent him again, a very short and polite email, asking him if he could answer my email since I am defending my prospectus within the next few weeks and I need his opinion on it. Again, I didn't get an answer. It is really irritating. I don't know how to ask him if something is wrong! Due to this new virus, the university is closed, and the only way I can reach him is through email. What should I do? I've been thinking of changing this supervisor but his field of study is so close to my topic, and he is really knowledgable that I regret missing him :(
Your situation has the first thing I would try built in - ask your other adviser for help. Your committee probably communicates with each other, even if they're in different departments at the same school. Tell the adviser who is responding that you're held up because the other adviser hasn't replied, and see what they suggest. If they are indeed in contact with them, then they might be able to nudge a reply. They might also be able to inform you of anything going on that might be causing this, like an illness.
If that doesn't work, or if your responsive adviser is so far removed from the unresponsive one that they can't help, go to someone else in your department. Do you have other junior committee members that might be able to help? What about a grad student adviser?
The bottom line is that, while professors being bad at replying to emails is sadly common, it isn't professional, and you should keep at it. With as much politeness and tact as possible, of course, since this person has a lot of power over your future here.