BTW, I have built a 4x5 vertical tank developing apparatus for my sheet film. It holds 4x5 for 1 liter of developer. I have adjusted times to meet my criteria. BTW. those times are different for 135 and still different for 120 "in a can" developer! sheet film is sheet film, roll film is roll film. ROLL WITH PUNCHES is my advice. all things are not equal. much to my ( and others) chagrin. unfortunately you can not set it and forget it!
AS to the OP, it still may be true that your developer is not what it appears to be, ( in good health) . Make sure your chemistry is top-notch ( no screwin' around with garbage) Also, Pinhole- if reciprocity failure is not accounted for, then you may suffer from "under=exposure". I would try FIRST. using reg. film, in reg situations, comparing them (135,120,sheet film) making careful notes. make those adjustments, verify ( re-shoot) nail THAT, THEN.. . . . move on to something like pinhole, ( i.e.is your pinhole exactly the f-stop you think it is?, larger or smaller) that will effect duration of exposure i.e. C.I. Also L>F> pinhole make sure you including Bellows factor!!!!!! and EVERYTHING ELSE. while you are at it. Clean out the darkroom. ITS only good Karma!!!!! Also if you are having problems? look towards things that you need to recenter yourself spiritually too. this effects and affects Everything!! I always ADD more Phen shwey!!! although I probably spelled it wrong. Good Luck. May the Schwartz be with you.