K3 Downloading while asleep

[fisab]

Apologies if this has been covered before.

I have a K3 (Keyboard). While it is in sleep mode it doesn't download articles which have been emailed to it.
My wife has a K4 and it does download those articles.

Example
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Both kindles in sleep mode all day and WIFI is on in the house. A periodical is emailed to both at a scheduled time during the day.
I turn the house WIFI off before we go to bed. Then when I 'wake' my K3 the article does not appear. When my wife 'wakes' her K4, she has immediate access to it (even though there is no house WIFI active)

Is this an extra 'feature' with the K4 that my K3 doesnt have.

Many thanks in advance for any help on this.

(Note: the article does download to my K3 when I wake it while the house WIFI is active).