Milk Stout bottles flat. Pop caps and add yeast to carbonate?

In February I brewed a delicious Milk Stout. I bottled and waited patiently. Sadly, after several weeks they were flat. I waited. Still flat. I waited more. Same thing. I'm not exactly sure what went wrong. I bottled with ~3.61 oz's of corn sugar and let set in an appropriate place. I've never had issues in the past, nor since, when bottling.

I still have a lot of bottles left. I'd like to get them carbonated.

I was thinking I'd hydrate some yeast (I used US-05), pop all the caps off, add some yeast, recap, and shake them up.

Thoughts to this approach?

My challenge is I'm not entirely sure how much yeast to add. I also don't know if I should add sugar too, or if I should just add sugar.

I'd love to hear your thoughts.

Whatever you do definitely do not shake up your beer! That will aerate it and it will taste bad. With fresh yeast you shouldn't really have to do anything to get them active. Especially if you're adding sugar.

With that said, if it were me, I'd taste a little of the flat beer. If you can taste the sweetness of the sugar then I wouldn't add anymore to it. If not then I'd add a little bit of priming solution to each one before you add the yeast. You can also look up carbonation drops too and see if those might work. I've never used them but I hear they do work.

ABV and bottling date?

are you sure you added sugar and not lactose at bottling?

For 5 gallons, 3.6oz isn't a lot of dextrose. Definitely shouldn't be completely flat, either.