Phone won't update apps on WiFi

[skunkfunk]

Hello people,

I don't know if this is a problem but it is for me. My phone won't update apps automatically when it is on WiFi. It starts to update when I turn on mobile data. As you all agree that updating via mobile data is not recommended due to costs. All of my apps are stated "allow auto-update" on the store and it is also clicked in the store's settings.
What can I do to fix this? My android is 4.1.1.

Thanks