If we equate "will" to "perserverance" then we may have struck upon one of the major ingredients of success. Yet, as the posts above have pointed out, there are many factors that can make success more or less likely for an individual. Access to resources (money, education, etc.) can go a long way to determining one's ability to succeed.
However if we focus on the potency of will-power as a facet of success, there are plenty of examples to draw from.
Thomas Edison is famously quoted as saying that in failing in his first 999 attempts to design a working light bulb, he was really building up to his one success. He also said, "Genius is one percent inspiration and ninety-nine percent perspiration."
I would say that Edison is a nice example of success and will-power's role in achievement.
A lot of people are successful in the areas that interest them. That's because they're willing to do the work in those areas. You might have somebody who is unsuccessful in school, but incredibly good at a certain video game--they are willing to do the work to become successful at that game because it is interesting to them. We just don't consider them a "success" because it is a video game that they are successful at.
Some people are more successful academically, even though they are not the smartest kids in the room, because they are "interested" in the end result, which is academic success, even if they aren't interested in history or math or physical science, etc.
I, too, am not convinced that will is the sole factor that determines success or lack thereof. However, if you want to write an essay advocating that position, a good way to begin is with an anecdote, meaning a little story that makes your point. It need not be a true story, but a story about two people, one of whom has will, contrasted with one who does not have will. The results for each will make the point you wish to make, and then you can go on to support your position in various ways, with various studies, perhaps, or statements from experts.
It is certainly true that effort and will matter, but they sure aren't everything. A person who is born with a lot of intelligence and a lot of money will be a "success" without having to try very hard while a poorer person will have to work harder to succeed. It's not just will power...
Leaving aside the question of what defines success, I would say that it depends largely on the person and their situation. Sometimes it is a lack of will. But sometimes, I think, it is also a lack of knowledge. If that wasn't the case, then why bother with education? To some extent, in my opinion, it is a matter of circumstance. Having taught amidst dreadful poverty for much of my career, I have seen many students who faced almost impossible odds, no matter how much desire they had. The will to overcome adversity or whatever one faces is indeed important, but I think the idea that all who have not met with success in life lacked willpower or desire does not take certain realities into account.