Have you ever repotted it? If not, it may be time. I’d lift it out of the pot and check the roots.

tj

Lack of light and/or over watering would be the most common reasons.

its not uncommon for these to shed the oldest lowest branches ... on some level.. its normal

understand these are forest trees in their native area ... though they are commonly grown as houseplants ... its not easy to keep them perfect ... this their future potential when really happy...:

https://duckduckgo.com/?t=ffcm&q=Araucaria+heterophylla&iax=images&ia=images

it is a tree .. so err towards less water ... i seem to recall that the parents watered theirs once a month.. if that.. and it was in front of a furnace register for 5 months a year ... it didnt look as prime as yours.. but it wasnt much worse .. my point being.. perhaps you are doing just fine.. and there is nothing to worry about ... lets try not to harm it with too much love ...

is there any reason it couldnt go outside in full shade for summer???

ken

ps: what are you going to do with it.. when its 2 feet taller than the ceiling ... lol ...