No fiz root beer

The recipe is just too simple to mess up but I have twice now. My root beer is flat. I saved it and got it back down to room temperature. How can I fix it. It tastes fine except for no carbonation.

Oh, I forgot to mention I used Hodgson Mill yeast. The recipe said you can use any yeast. Could that be my problem?

Please post the recipe and instructions.

We need more information about your process, particularly when it comes to carbing. How do you add the yeast, how long is it left out to carb, are you bottling, glass or plastic, kegging?

1 cup sugar in a plastic liter bottle then I added 1/4 tsp of Hodgson Mill yeast and shook it really good then swirled it to make it concave then added 1 tblsp root beer extract. I added water to about 1/2 of bottle and swirled it to dissolve everything. I filled the bottle up to an inch from the top just at the neck line. I let it sit at room temperature for 4 days. The bottle felt pretty firm so I put it in the fridge. I let just a tiny amount of air out (very little) and the next day I tried it and it was good but flat. In fact it was really flat.

Hmm, sounds about right. When I used to bottle carbonate in plastic bottles it usually took three to four days, be sure that the bottles are hard though. I'm not sure what you mean by pretty firm. I waited till mine were good and hard. Also, when did you let this tiny amount of gas out? I would leave the bottles in the refrigerator for at least 24-48 hours for the gas in the head space and the liquid to equalize before opening them at all.