I'm trying to set one of my lists to "add words", but it automatically keeps going back to "review only"

Is "add vocab from" pointing to a section in a list that still has remaining words to study?

when I adjust the list to "add words", I then see it the list in the section that says "adding words from". Then I go the queue and it won't let me click on the plus sign to add new words. I go back to the previous page and see that the list has automatically been reset back to "review only", even though there are lots of words that have been done yet

Look at the words in the list itself. Are there actually any new words, or have you already added everything? Look at the actual section from which you are trying to add.