I saw that many people have my problem and that there are no guides about it.
I had found all the wells but it still did not give me the achievement.
I solved it like this...

First of all check in the large well in the center of the HuB maps of the levels and (ATTENTION) count one by one if you have all 14 wells. They are not in numerical order so if at the end one is missing it could also be one of the first. Count them and make sure you have them all.
If you are missing one you can always click Select and see in the levels if you have the well for that level.

If you have verified that you have all the wells you must be sure that you have also completed them to the end.

I had all the wells but I had not completed well 4 to the end because I had not recorded the time and the coins. I completed it and received the achievement.
I hope I was helpful.

Happy platinum hunting.