I have been sending villagers to all 4 flowers for the last 2 days, but haven't gotten the "you understand this plant" or whatever the message is supposed to be.. so they get better at healing, but I can't complete the puzzle. Any advice?

make one villager into a doctor...that one will generally learn all four plants quickly

I've tried that, but it still isn't working.. I had an adept dr and a master dr working on them, and the only one that I've been able to get is the cactus.. it doesn't want to give me the other 3.

Just keep trying. I had different villagers examine the different plants - just make sure you keep sticking a villager at the plant until you get the message that they learned about the properties of it. Sometimes they just shake their heads and walk away and if they're highlighted you'll be told they weren't successful this time.

Good luck with the plants.

No need to be doctors to do that puzzle, I always do that in the beginning of the game. Just do it on the right plants.

in my experience if you get them to understand the cactus first for some reason that makes all the difference

Thank you everyone for your advice, I finally got it! much appreciation to you all!