Hello hsintek,
Thank you for visiting Apple Support Communities. It sounds like you're not receiving the banner notification for Ask to Buy. I'd be happy to provide information that may help.
To start, restart your iPhone to see if that helps: Restart your iPhone, iPad, or iPod touch.
Next, make sure your notifications are set up properly: Use notifications on your iPhone, iPad, and iPod touch.
Take note of the section about Deliver Quietly. These notifications appear in the Notification Center, but don't show up on the Lock screen, play sounds, or show a banner or badge icon.
Also, check to see if Do Not Disturb is enabled: Use Do Not Disturb on your iPhone, iPad, and iPod touch.
Lastly, be sure that you're still signed in to your Apple ID under Settings > tap on your name > iTunes & App Store.
I hope this help helps. Take care!