Avocado Trees Loosing Leaves and Dying?

I've had two avocado pips go from a glass bottle to a small pot to a large pot, separately, since August 2019. They have been thriving - growing rapidly from the second their roots came out the pip.

All of a sudden, recently, they are starting to look down. The larger one has leaves that are starting to become almost hard, and the smaller one has no leaves left - I pulled them off as they were all dead and discolored.

I'm really desperate to save them, but don't know what is wrong. So here's a few things I've noticed:

- The first time I put them in pots, I used normal compost. The second time I put them in larger pots, I used potting soil and mixed it with sand from the garden. Both large pots do have holes in the bottom.
- The smaller plant started to look down before I repotted it so I thought moving it to a larger pot would help. It did not.
- I've never grown avocado plants at this time of the year. It's autumn here in South Africa, but it's not nearly as cold as other parts of the world so I don't think it's too cold for them or it's too cold to move them inside just yet either.
- I've moved one into the sun and one into the shade (it gets a few hours of sun in the morning).
- It's quite warm during the day still so I give the larger one a full bottle of water and the smaller one half a bottle when the sun sets in the evening, usually once every second day, unless it rained in between.

If anyone can provide tips or reasons as to why they think this might be happening, I'd really appreciate it!