New customers have a tendency to evolve through three phases once they decide to buy from you.
Initially they feel very excited about their decision, before going through a learning curve where they may struggle with blending in your products/services.
Finally, they begin to experience the value that you provide and the relationship settles down and finds its own balance.
Phase 2 can be a potentially vulnerable time for a sales person, because without the benefit of an established track record, and in the face of possible problems – no matter how minor, this is the time when most newly acquired customers are apt to change their mind.