Brewed up 15 gallons of 1.103 Imperial Stout 5 weeks ago. Pitched a good amount of yeast but probably should have used more. Fermentation went well for a week, then died down. I racked after 2.5 weeks and it was only at 1.040. Added a pack of S-05 and it's still fermenting but it's only down to 1.35 at this point. My pH is 4.6. Is this OK? I plan on leaving it for another 2 weeks or so at the minimum but was wondering if the pH was slowing things down. I expected the SG to be much lower after 5 weeks.