I submitted a paper to Elsevier almost two months back. After 15 days I got a mail that Paper has been sent for Review but the status continued to remain "Editor Invited". After further one month, the status changed to "Reviewer Invited". After ten more days i.e around two months from the submission date, I got a mail stating "I have received all the required reviews, which I will now evaluate before making a decision " and the status changed to "Ready for Decision". After two more days the status changed to "Under Review." I am not able to understand the status of my paper. The mails I got were sent from EviseSupport@elsevier.com
The editor received the reviews, but there was some problem that meant that they could not make a final decision about acceptance. Usually, when all the reviews are in, it is not too difficult to reach a decision, but there are plenty of cases where there is real difficulty. The simplest explanation is that the two (or more) referees disagreed, and the editor did not feel entirely convinced by either of them. That would mean some further evaluation of the paper is required. If the editor is sufficiently knowledgeable about the subject matter, they might evaluate the paper on their own. They might send it back to the same referees, to see what they think of each other's comments. However, the most likely alternative is that the editor sends the manuscript out to one or more new referees, and that is probably what happened to your paper.
(There are other situations in which additional review might be needed, but they are less common. The editor might feel that a referee report is clearly too carelessly done or otherwise unsatisfactory. The referee might submit a carefully reasoned report, but be unwilling to make a judgement about whether the paper is suitable for publication. In any case, the editor's response is probably going to be the same; they will most likely send the manuscript out for another round of reviewing.)