Meeeeh, it's probably best to err on the side of caution. Even if you do add the tabs, you'll be losing a lot of CO2 from the headspace (i.e. that nice "psssht" sound you get when you crack a bottle open) so you'd be wanting to add more like 2 tabs anyway, which seems even more dangerous
But what exactly do you mean by faulty thermometer undercarbed your batch?
Also, you may also not have given it enough time. The general consensus here is that the minimum amout of time you should give your bottles to carb/condition is 3 weeks stored at 70*F. This is also only for average gravity beers.... heavier beers will take longer. If after 3 weeks at 70*F you're still flat, then you should start to think of solutions.