They're all "our/ours" but since the possessive is an adjective, it must agree with the thing that is possessed.
nostro = our masculine thing
il nostro libro -
il libro è il nostro
"our book" - "the book is ours"
nostri = our masculine things
i nostri libri -
i libri sono i nostri
"our books" - "the books are ours"
nostra = our feminine thing
la nostra mucca -
la mucca è la nostra
"our cow" - "the cow is ours"
nostre = our feminine things
le nostre mucche -
le mucche sono le nostre
"our cows" - "the cows are ours"
It would help to know what exactly is going on here. First question: Are you 100% positive you didn't make any kind of typo? Adjacent to that, the course contributors have no control over the correction algorithm. That's programmed by the devs.
Past that, sometimes Duo glitches.
Past that, the course contributors need to manually enter all of the different answer possibilities into each sentence's database individually. That is a much greater undertaking than you might think. Because of this, it is possible that there are oversights in a given lesson or inconsistencies between lessons.
If you can demonstrate that your answer was correct/valid/typo-free but it was still marked wrong, you can flag it and report "My answer should have been accepted." But first make sure your answer was actually valid and not just an assumption you're making by imposing English grammar onto Italian or misunderstanding how Italian grammar works.
Patrick: Yes, l'ape is feminine and "ours" agrees with it in gender, so feminine. It doesn't matter whom "ours" refers to. Think of it this way. How would you say: It is our bee? You'd say E' la nostra ape -- regardless of whose bee it is, males, females or a mixed group. The possessive agrees with the noun it modifies. Another example: It's our car. 2 men = E' la nostra macchina. 2 women E' la nostra macchina. Husband & wife: E' la nostra macchina. So again, the possessives agree in gender & number with the noun they're describing.
Both are the exception to the general rule regarding how nouns end:
-o | singular masculine
-i | plural masculine
-a | singular feminine
-e | plural feminine
Nouns that end in -e in the singular and -i in the plural are irregular. You need to memorize whether each individual word is masculine or feminine.
You can't translate a complete sentence into a noun phrase.
L'ape è la nostra. = The bee is ours.
"L'ape/The bee" is the subject
"la nostra/ours" is the possessive pronoun in the predicate.
La nostra ape = Our bee
A simple noun phrase with the possessive adjective directly modifying the noun in the same phrase.
That's just how Italian grammar works. Except with singular unmodified family members, you need to have a definite article before the possessive adjective.
With the possessive pronoun, it can take it or omit it, but it shifts the emphasis a little bit.
"La gatta è la mia" means "The CAT (and not something else) is mine."
"La gatta è mia" means "The cat is MINE (and not someone else's)."