Why aren't primidone's lactam groups acidic?

I understand that 5,5-disubstituated derivates of barbituric acid only show lactam-lactim tautomerism opposed to the lactim-lactam and keto-enol tautomerism of barbituric acid, hence the 5-mono and 5,5-disubstituated derivates are less acidic. 


Primidone differs from phenobarbital in a C2 oxo group, so it's lactam groups are not acidic and it is not soluble even in alkali-bases. Why aren't the lactam groups acidic exactly? The oxo groups should still present quite a bit of electron withdrawing effect, shouldn't they?


[![enter image description here](https://i.stack.imgur.com/jrKCW.png)](https://i.stack.imgur.com/jrKCW.png)


