Not receiving Ask to Buy requests

It appears you have already perused: If Ask to Buy isn't working - Apple Support about looking for the request in Messages or Notification Center.

Make sure notifications haven't been disabled. This article discusses notifications: Use notifications on your iPhone or iPad - Apple Support

and this one Change notification settings on iPhone - Apple Support

Make sure the software is up to date on all devices involved.

Sometimes it helps to have the devices involved take a fresh look at settings:

● Try signing out of accounts and back in again.

Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support

Sign in with your Apple ID - Apple Support

● Restarting is a simple step that can resolve many unexpected behaviors.

iPhone: Restart your iPhone - Apple Support then choose your model.

iPad: Restart your iPad - Apple Support

Mac: Ways to start up your Mac - Apple Support

If a problem persists, particularly over several days, try contacting Apple for assistance:

Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

"See a list of Apple phone numbers around the world."

Click here --> Contact Apple for support and service - Apple Support

That resulted in the same thing I suggested in my second post except it also removed your other network settings too.

Read Return iPhone settings to their defaults - Apple Support

Reset Network Settings: All network settings are removed. In addition, the device name assigned in Settings > General > About is reset to “iPhone,” and manually trusted certificates (such as for websites) are changed to untrusted.

In fact the second solution I posted apparently originally came from Apple too, but it isn't a documented solution so it typically won't be "Recommended" here.

Here is a solution where editing the name of a mobile device resolved recent occurrences of this issue. It has worked for at least 9 people (and likely many more who never provided feedback after that solution was posted). It isn't something that Apple has in its documentation so I am putting it in a separate post for those who are interested in exploring beyond Apple's officially recognized solutions.

Go to: Settings > General > About > Name

Change the name

Then "Done" and retry.