Being born again is a status conferred on a member of certain types of Christian religions. If a person rejects the beliefs that led them to become born again, converts to another religion, or becomes an atheists, I would guess they would no longer have "born again" status in the eyes of their former religion.
I believe people who are "born again" can also lose that status. People always struggle with faith and when they do so, they often break the rules of that faith. Being "born again" isn't just something you declare and then walk away, it's suppose to be a change of life, a change in the way of thinking. For those that can't keep up their new life style, I believe they are in terms, unborn again.
I believe it is possible for a born again Christian to become unborn again. So, to speak, people who generally find comfort with faith, also at times, struggle with faith. I believe this is common for people, especially those affiliated with the Christian church. I do not believe it's an alarming or surprising problem.
We use an act of our will to believe, certainly needed to believe the Gospel. Unbelief is also an act of that same "will" which would lead to rejection of Christ, which would lead to loss of "Born Again Status" and our salvation.
Belief or unbelief is the most serious and consequential decision we will ever make. ALL of us must make that decision at some point!
Once a person is saved, the person is saved for a lifetime. It is possible to backslide, but backsliding does not require full on salvation because, theoretically, God never left. It is necessary, however to repent sins that happen during a backslide in order to renew your commitment to God. It's similar in nature to renewing wedding vows. You don't become unmarried, but you do renew your commitment.
This is a very controversial topic, so even though I'm answering "disagree," I realize that I could be wrong. In the traditional Christian belief, when someone becomes born again, it is a very spiritual experience where they really feel the Holy Spirit speaking to their hearts and are changed. Because of that, if the experience was truly genuine to someone, it's difficult to believe that they could really lose the experience altogether. However, some people may not have a genuine experience and only believe they did, or they may walk away from the faith.