Q: Family Sharing "Ask to Buy" Issue

I'm having the exact same problem.  I posted my question here:

family sharing / ask to buy failing by repeatedly asking for password

I'm getting the impression that family sharing / ask to buy doesn't work at all. 

Having same trouble. First they lose the fingerprint approval and now I can't approve purchases for my kids.  And to make matters worse, looks like APPLE REPS have fled these forums.....

Can anyone help!?

I don't know about you, but I'm sick of seeing these forum posts unresolved by anyone who actually works for Apple. I've had this issue on and off for months now, no rhyme or reason to it. Why put a feature in the product, if it doesn't work correctly? My mac is fully up to date with 10.10 (I can't install 10.11 for company reasons - this is a corporate laptop) but why won't it ever take my password? It works fine for FaceTime, iCloud preferences, calendar, mail, etc, but not the family sharing approval window notice that comes up...

The solution is hidden at Request and make purchases with Ask to Buy - Apple Support:

If you decline the request or don't make the purchase, it will be deleted after 24 hours, and the child will need to make the request again.

So what's going on here is that you're trying to approve an expired request, and it's failing, over and over, without notice or explanation.