Clyde had great success with zinc dust, if the bark was too old. After time dmt oxidizes into dmt-n-oxide, which is insoluble in naptha. When u add the zinc dust into the acid step (clyde has only read about it being used with acetic acid, he doesn't know about any other) and shake it for two hours, the zinc dust snatches the oxide molecule from the dmt-n-oxide turning it back to dmt.
Or you could use another solvent, like xylene, which the oxide form is soluble in, if old bark is the problem.
All posts by clyde frog and his affiliates are fictional meanderings of the imagination of a fictional 8 yr old boy from a certain adult fictional comedy cartoon.