Rate of fermentation, or turbidity of fermentation is really irrevelant, as is things like size of krausen, or amount of bubbles in the airlock. The only thing that matters is whether you have fermentation or not...simply that.
If you have a decrease in gravity you have fermentation going on, that's all that matters. Since often there is a 72 hour lag time while the yeast reproduces enough yeast to do the job, and you are at 96 hours, then you may have only had 24 hours of fermentation actually happen
so for 24 hours of work, going from 1.043 to 1.036, is perfectly fine.
That's why things like the 1-2-3 rule are useless, because they don't factor in things like lag time. ANd if people impose a timeline like that on them, then they may be rushing beer off the yeast and into secondary way too soon.
You have to remember that you are dealing with living micro-organisms
and they have their own timeframe, their own agenda. We are not the boss here, they are. They've been making beer since long before our great great great grandfather copped his first buzz from a 40 of mickey's out back of the highschool, so they are the experts, and they will finish their job, when the job is done.
That's why many of us just walk away from our beers for 3-4 weeks, skipping primary, then we bottle...so they can finish the job, on their terms.
You really don't need to add more yeast or do anything else.