When I do
wnl.lemmatize('promotional',wordnet.ADJ), I get merely
'promotional' when it should return
promotion. I supplied the correct POS, so why isn't it working? What can I do?
"promotional" is not an inflected form of "promotion", therefore "promotion" is not the lemma of "promotional". Actually, "promotion" is a noun and "promotional" is an adjective.
Maybe what you actually want to do is not lemmatisation but stemming. Note that the stem is the root of the word and, certainly, the stem of both "promotion" and "promotional" can be "promot" (or "promotion", depending on the convention).