Welcome to Fluther.
This is a question that just cries out, in a very faint, plaintive, sort of wailing voice from way out in the wilderness for “detail”. I.e., what business are you in, who are your clients, and why is it vital that they reply to your emails?
I work in the home office (well, the US home office, anyway) of a multinational engineering-construction company. I deal with our field management people in jobsites all over the world (currently all on “the other side of” the world). When I write emails to them I almost never expect to “hear back” from them; I only want them to do something that will enable them to continue receiving service from our IT people, make crucial upgrades in their computers and messaging systems, be able to continue filing expense reports, and such like that. It hardly matters to me whether they write back unless my instructions are unclear, the recommended actions cause or will cause problems, or something else is wrong.
When I need to get a response, such as “What is your computer name? The IT people have to know, and I don’t have it in my own records,” then I write that every day until I do get a response, or I call them at home, when my day is just starting.
Contrary to @fremen_warrior‘s advice, I suggest you make the follow-up emails very short. “I need an answer to yesterday’s / last week’s question. Please respond.”
But not knowing anything about you or your business, I’m wondering why it’s so important that your clients reply.