Trying to apply for a Phd opportunity, I find it's really hard. Some prof. answer my email soon. Some NEVER! Did u think that's normal?

Most supervisors will ignore applications from people who do not qualify for funding and have no means of funding themselves. This is because they receive such enquiries every day and do not feel they have time to reply.

When you write for a position where you do not qualify for funding make sure you let them know that you have your own funding or expect to find it. Otherwise restrict your applications to those with funding available to international students.

Yes, I think those professors are too busy to reply to each request.

It's sad but true. They are busy people. It's hard enough getting their time when you're there in person !

That is assuming they are there!

See if you apply in bulk to all those who are from your field, your chances of getting a reply fade. You must do a proper preparation before e-mailing to any faculty.
I guess you know it and if not tell me i will let you know. But never send bulk e-mails, Professors are very experienced and they show interest in only those who have genuine interest in their work. Even if they don't have funding, reply is mostly assured.
ALL THE BEST

sad but don't lose hope. Guess what even some lecturers who are my teachers they won't reply me, obviously not for phd query but for small module queries.

on the contrary, I agree that most professors are very busy and secondly they received many emails daily. I always send emails to lecturers and not to prof and mostly i received rep even negative response but with good advice and encouraging comments.

I encounter the same problems as you.Futhermore i'm not UK citizen,i'm chinese, so i want to apply a phd position in UK is more difficult.