I have one villager who is an Adept Doctor. He has been working on studying one of the 6 plants for about 100+ tries now and I still have not received the message that the plant has been mastered. These are my questions:
1. Is it possible to master a plant by revisiting it over and over?
2. Do you need to keep working on one plant exclusively or is it better to rotate between plants (aka. which is faster)?

well if you keep dropping him on the same plant you have to wait till it says "you have improved in healing" or whatever the message is. You dont master an individual plant or all of them. You just use them to incrase your healing skill until you are a master healer. Doesnt matter which ones you use they all have the same affect on skill increase

You should master the individual plant within one successful visit. If you still haven't gotten the herb mastery puzzle completion message, my guess is that you've missed a plant. Sometimes, a villager will 'fail' in his study of the plant and you don't notice if you've been distracted elsewhere. Try making a circuit of the plants again and make sure you've gotten them all.

Keep that villager in focus, drop them once onto a plant. Wait til they finish with it and walk away. If you don't get a message, your tribe has already mastered that one.

Huskergolfer, is it possible you missed the message the first time? Once you get that message, it won't come back again.

Oh and did the village celebrate the understanding of the plants?

oh i see for the herb mastery puzzle...geez brain fart. Its a long bad day today

That did it...forgot one...thank you very much!