I used to have the same problem. Have you tried 2-3 packets in 40L without making a starter (I didn't think you were meant to build starters with dry yeast)? Have you also tried building a starter with liquid yeast? I buy liquid yeast and overbuild the starter, 2/3 goes into the pending brew and 1/3 goes into a sanitised jar in the fridge, then I start the process over again for a few brews before resetting with fresh yeast..
Also, I build 3 - 3.5L starters for ~26L brews (remembering I save a 1/3 for a jar) - you might want look at your starter sizes (depending on the OG of course).. lots of things to try but it's going to take you a few brews to work it out!
Also, I build 3 - 3.5L starters for ~26L brews (remembering I save a 1/3 for a jar) - you might want look at your starter sizes (depending on the OG of course).. lots of things to try but it's going to take you a few brews to work it out!