When you say a "couple of weeks" just how many weeks do you mean?
First, The
3 weeks at 70 degrees, that we recommend is the
minimum time it takes for average gravity beers to carbonate and condition. Higher grav beers take longer.
Stouts and porters have taken me between 6 and 8 weeks to carb up..
I have a 1.090 Belgian strong that took three months to carb up.
Temp and gravity are the two factors that contribute to the time it takes to carb beer. But if a beer's not ready yet, or seems low carbed, and you added the right amount of sugar to it, then it's not stalled,
it's just not time yet.
Everything you need to know about carbing and conditioning, can be found here
Of Patience and Bottle Conditioning. With emphasis on the word,
"patience."
Additionally, A larger volume sized bottle usually needs more time to carb AND condition. I have some pints, 22 oz bombers and other sizes that I often use, but since I enter contests I usually also do a sixer or two of standard 12 ouncers for entering. And inevitably the 12 ouncers are done at least a week faster than the larger bottles....some times two weeks ahead of time...
Also the rule of thumb is 3 weeks at 70 degrees for a normal grav 12 ounce bottle....to carb and condition....It takes longer for the yeasties to convert the larger volume in the bigger bottles to enough co2 in the headspace to be reabsorbed back into the solution...A ration I don't know how much...
Big Kahuna gives a good explanation here...
Simple. It's the ration of contact area just like in a keg. The c02 will need to pressurize the head space (Which takes LESS TIME) in a bigger bottle (More Yeast and sugar, roughly the same head space) but then it has to force that c02 into solution through the same contact area...thus it takes longer.
The
3 weeks at 70 degrees, that we recommend is the
minimum time it takes for average gravity beers to carbonate and condition. Higher grav beers take longer.
Thirdly Cooper's drops tend to take longer to be broken down by the yeast than dissolved priming solution does. Usually a week or 2 longer than if they had been primed normally.
I would til the beer's been in the bottles around 5-6 weeks, before messing with them. If you add too much sugar and the beers have simple not carbed yet, and there is residual sugar still in there then you could over carb you beer and end up with bottle bombs.
It's always better to wait to make sure the original priming sugar did get used up before adding a random amount that you don't know how much the combination will raise the level of co2 in the bottles.
This guy "THOUGHT" he had a problem, but it just turned out that he was expecting them to be carbed sooner that they were, if he hadn't listened to me, he would have had bottle bombs as well.
https://www.homebrewtalk.com/f39/i-think-i-primed-maltodextrine-269636/