As far as the Cooper's carbonation drops, I've had my issues with them but it's always been over-carbonation and/or inconsistency. I've never had them fail completely.
I would give it time and warmth. Unless you've already done that.
+1 on it
not being the carb drops. I've heard of other brands having issues, but the Coopers ones have been rock solid reliable for me.
The big questions are how long they've been carbonating and at what temp. Since the Coopers drops are simply a ball of candied sugar, it will dissolve more slowly at lower temperatures. I usually carbonate at 70ish degrees (F). A day or two after bottling you can often still see the sugar drop in the bottom of the bottle. I usually invert the bottles gently a few times every day or two for the first week if I think of it. I think that helps speed the process along when using something like carb drops.
I give big beers at least 3 weeks before I even bother checking carbonation, and it can often take a month or so at room temp before they reach the level of carbonation I'm shooting for.