refill vs fill again

refill is a noun but fill again is not a noun.

Word	Noun	Adjective	Verb	Adverb
refill	Yes	No	Yes	No
fill again	No	No	Yes	No

As verbs, refill and fill again are synonyms defined as:

Other synonyms of refill include replenish.

refill (noun)	fill again (noun)
a commercial product that refills a container with its appropriate contents
a prescription drug that is provided again

refill (verb)	fill again (verb)
fill something that had previously been emptied	fill something that had previously been emptied