I have been using a stout recipe that uses wyeast 1728 and has a OG of 1.058 and a FG of 1.036. It's only roughly a 2.6 ABV. The next time I used this recipe, I added more grain and raised the OG to 1.064 to raise the ABV but my final gravity on that batch ended at 1.042 for the same 2.6%. I've done this a few times and the pattern follows. Do I need to just use a different yeast?