Typically, it is not better product knowledge or more training on their sales process. These are the basic tickets to entry. What separates the successful from the unsuccessful is courage and how they spend their time.
As a sales person, how you spend your time and who you spend your time with is totally within your control. Do you want to spend it with a customer that only has the potential to buy a small amount from you or do you want to spend it with a customer that can buy a large amount from you?
Many sales people are intimidated to meet with big customers. The reality is the buyer at a big customer is the same person at a small customer. That buyer likes to do the same things in their free time, they love to talk about their families, their favorite hobbies and teams, and they have the same issues and problems at work and home. The only difference is the quantity that they order.
It is your choice who you spend your precious time with. Be courageous!