In Chapter 18 of Emma by Jane Austen, what is the cause of disappointment for Mr. and Mrs. Weston?

As
Chapter 18 in 's  begins, readers learn that Mr. Frank Churchhill has not
followed through with traveling to visit his father and stepmother, Mr. and Mrs. Weston, at
their country estate. While Mr. Weston is at first disappointed that his son has not kept his
promise, he quickly justifies to himself that "Frank's coming two or three months later
would...

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