I couldn't be arsed with the duration of oral...smoking is perfect for me, it's why i cba with shrooms or sid any more either...
I dunno Ismene, you've probably been doing it longer than me anyway

....I make sure everything is warm, hot almost....first pull i stir the solvent a few times over 45 mins, like yourself...
2nd pull a couple hours
3rd pul overnight.
4th pull maybe leave it for 3 days or so, on a radiator, stirring a few times in between...
I use the same solvent for each pull
freeze precipitate each one.
And then at the end evaporate the solvent thats been freeze precip'd....
Maybe your bark is a different source....another friend I know has similar results to me with bark form same source...
Or maybe it's the solvents? i have a mate who works in a motor body shop, and has access to all sorts of fancy panel wipes and thiners and solvents...he showed me the brand names etc and I got one that's nearly all xylene (fuckin nasty shit to evaporate), another panel wipe is made up of nearly all heptane (that makes a nice white crystal), and another totalfuckin cocktail of solvents that I sometimes use as a final "mop-up" pull, that yields red-brown jungle spice.....