Why is April concerned about the empty mailbox, and then later concerned about Dorthea's lattest letter in The Egypt Game?

Without a more specific chapter reference,
this question requires some guess work.

April's mother dreams of being a
Hollywood star, and she has sent April away in order to pursue that dream. April believes that
the situation is temporary; however, she starts to worry as her mother's letters do not show up
with any kind of frequent regularity. This lack of communication from her mother really starts
to worry April because April worries that her mother is beginning to forget about her. The
longer the mailbox goes empty of letters from her mother, the more April worries.

Regarding the latest letter that the question mentions, I think it is asking about the
letter from her mother mentioned in chapter 13. The letter tells April that her mother has
married and moved in with Nick. Her mother also writes that she is sending April the rest of her
stuff.

This letter devastates April because it confirms that she will not be
joining up with her mother any time soon. April tears the letter up and flushes it down the
toilet. Fortunately, by the end of the book, April no longer feels a huge draw to rejoin her
mother, and April actually refuses the invite from her mother that finally shows up in the final
chapters of the book.