I have a paper which was "under review". Then, it has become "reviewers assigned" for some time.
Does anyone has similar experience? One of the reviewers declined review?

Can anyone help MeaninginLife?

Irrespective of what it means... You can't control the process so don"t worry.

The terminology may be different between publishers/ systems.

Generally, editorial staff will look at it first to ensure it is formatted correctly.
It will then go,to an editor, who will decide if it is worth sending out for peer review. This is probably what 'under review' meant.

Reviewers assigned means that peer reviewers have been nominated and accepted. People may decline to review for many reasons - eg they are going on holiday. Peer reviewers do it in there own time, often last thing at night so it can take some time to complete. I usually have a quick,read when I receive the invitation. If it is very straightforward (obvious accept or reject), i will do it there and then, else it goes to the bottom of a list.

It has changed to 'under review' again. In this journal, the editor may send a paper to at least four reviewers.
Perhaps if one of them declined to review for various reasons, it would be 'reviewer assigned' again.

I wouldn't keep checking, put it out of your mind. It can take up to 6 months before you hear back about a paper or longer. You should wait at least three months before you email the editor about the status of your paper (I took a publications workshop which was run by an editor of a journal and learned heaps about the process!).

An editor will generally have a quick look at the paper to determine whether it's worth sending out, as Thesisfun has suggested. Then, it'll get sent out to the appropriate reviewers. They may accept it at first and then decide not to review it later, or perhaps can't review it. It can be difficult to find reviewers as they need to be subject experts.

For example, a paper I had submitted was sent to my supervisor for review! He of course had to turn it down and it had to be sent to someone else. He knew it was mine because he had had a look at it before I had submitted. Sometimes this happens, especially if your supervisor is a subject expert in your field. This meant that it took a long time because they had to find someone else to review it who was an expert enough to provide a sound assessment.

I had submitted at the end of July (2013) and didn't hear back until mid-Jan 2014 that it had been accepted. Then it was a crazy 5 months of revisions etc because they wanted to get it into a May issue.

So be prepared (if accepted!) to be expected to complete revisions in a quick and timely fashion!

Hi hoangnam,

Just received the verdict: revise and resubmit with major revisions.
Reviewers' comments are more than 4 pages long.

Dear all;

I have a paper with a status was "under review" on May 25 (2015). The status changed to "reviewers assigned" on Sep 2. The current status is still "Reviewers Assigned" (two months ago).

PLZ, can anyone help me about how can I ask the editor regarding this change...

Thx

Don't (yet). The posts above explain the process and the situation you find yourself in, thoroughly.