I have never used additional yeast to bottle, I just add the recommended amount of sugar.
Explain a bit more of your process. You shouldn't need to add additional yeast.
Can corn suger go bad? Your process seems pretty sound.
Does the beer have any residual sweetness when you drink it, does it taste like not all the sugar was converted?
70* ferment wrapped in blankets seems a little warm too.
When you say "wait for yeast to flocculate" what is the avg timing here?
Have you tried not racking to secondary?
Are you sure the corn sugar is well mixed in the beer at bottling and not stratifying?
Is your FG in target for your recipe?
Man .. your steps are identical to what I do and the first week(then 2 days in the fridge) I generally have light carb and then week two its good to go. Here is some examples. May be on to something with the old sugar though. I would also look at your capper and make sure something is not wrong with it.
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