Still flat

My second all grain is still flat after bottle conditioning since 3/25/11 at 70 deg.F. My last 2 batches were fully carbonated after 10 days Can I pop the tops and add like half a teaspoon corn sugar to each bottle? This batch was in primary for four weeks, when the water in the double bubble leveled out, I checked gravity and bottled with 3/4 cups corn suger for 5.5 gallons. Final gravity was 1.002, OG was 1.075. I pitched 2 packs of Nottingham. I know it takes a while to carbonate a higher gravity beer but I would think it would have at least carbed a little by now. Any suggestions?

There's nothing you need to do except walk away.

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."

If a beer isn't carbed by "x number of weeks" you just have to give them ore time. If you added your sugar, then the beer will carb up eventually, it's really a foolroof process. All beers will carb up eventually. A lot of new brewers think they have to "troubleshoot" a bottling issue, when there really is none, the beer knows how to carb itself. In fact if you run beersmiths carbing calculator, some lower grav beers don't even require additional sugar to reach their minimum level of carbonation. Just time.

In addition to what Revvy said, you are chilling them for a while before you try popping one open to check on the carbonation, right? Some of that CO2 is sitting in the headspace and will gas out if it's too warm (also, your beer will foam like crazy when you pour it, further degassing it). It needs to be chilled and allowed time to dissolve into the beer.

Definitely check out the link he suggested. Like he said, patience is a virtue!

Thanks guys, I apologize for my impatience, I don't quite understand why my first all grain, with similar OG and same amount of priming sugar carbed in 10 days, as did the partial mash before it(?)

Different yeasts work at different temperatures. Some yeasts floc out more leaving less to do the priming for you. And sometimes physics just takes some time off and really tries to screw with you.

Lots of unknowns and variables. I had one batch that wasn't carb'd after 2 months. I was certain I had not added priming sugar. I had taken a couple of gallons of the same batch and put it on oak for 2 weeks, and the oaked part of the batch primed just fine. It's probably 6 months now, and that initial batch is perfectly primed.