Apk Expansion File Downloading occurs frequently in anrdroid.why so happens?

Apk Expansion File Downloading occurs frequently in anrdroid.why so happens?

I've developed an android application using expansion library.
I've used required strategy to create and upload expansion file.I've
uploaded the apk and main expansion file in play store.And they're saved
as draft,not published.While I installed this apk and tested it in my
device sometimes after it being uploaded apk expansion file downloading
occurs.But when I tried after sometimes it shows "Download failed because
the resources could not be found".
I used only main expansion file.My app version code is 5 and expansion
file is 17509413 bytes in size.
My code snippet is as follows:
private static final XAPKFile[] xAPKS = { new XAPKFile(
true,
5,17509413L
)
};
Can anyone help me please?
Thanks in advance.