I'm doing freelancing.

One client hires me for 30 hours/week.
And I'm are getting work irregular.
Like
1 week is full of 30 hour
1 week just 10 hours.
1 week 20 hour.

What should I do?
I try to get other project but most of are For a full time.
One Client hires me but at that time the previous client gave me a full 30 hour's work so the new client could not get enough time and gave me low ratings because of time wasting.
I'm afraid that can happen again.

Talk to your client about it. Directly tell that you are doing the job in less time and you can do more, if possible.

Which site are you using?

It's not that hard. You just did not make yourself the simplest question : "If I expect I will get 10 hour work from the old client and take a job from the new client, but in the end the work for the old client is 30 hours, what will happen?". You go with the worst case scenario, that every week will be 30 hours, when you schedule new clients,and allocate the remaining time. You might have more time, but you can't promise the new client. In comparison, I am at university now, and I want to get a job. But I don't because I won't have time for both. If the time is not enough, what will I do? Leave university behind and pass less lessons, eventually staying longer, or do less work and miss deadlines? So I avoid the situation altogether.