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I have a hard time as well getting certain chemicals (namely green/purple), the problem is you don't get a lot of feedback on a subjects contentedness. They may dislike a certain thing but they don't really show displeasure well enough or at all. I have one custom character saved with nothing but turn ons and my character fits a good chunk of them, I do everything according to her turn ons and still only get 50% or lower satisfaction rating. Then, on the flip side I pick a randomly generated character with some turn offs my dude unfortunately checks off and I get 100% satisfaction right away. I seem to always NOT get the chemical/satisfaction I'm aiming for, then when I'm screwing around I suddenly have a large deposit of what I was missing.
I can't figure it out.
Having said all that...I've got more canidine than any other chemical. Make sure that:
-Your character is the gender/size your subject is attracted to
-Do whatever sex act they like
-Unless they like multiple orgasms only give them one then let them go (I'm not too sure on this one, someone correct me if I'm wrong)
That should hopefully get you all the canidine you need, failing that you can always go and exchange all your other chemicals in the node mini-game but you don't get a whole lot for what you put in.