How long has the tree been in ground?
I now see two years so young but the roots should be spreading out 3 feet or so.
Is it on flat ground, slope or hill?
If not on flat ground probably no possibility of standing water in root zone. On flat ground if standing water was ever possible during some high water event even a few hours root system could have been damaged.
Has the tree developed an extensive root system, of has there been some restriction, damage or dieback?
It's possible that during the tree's life some damage may have come to the root zone which has resulted in a reduced root mass.
What is the subsoil profile down several feet through the B&C horizons?
You may have sand on top but solid rock or an impermeable profile underneath. Maybe just a big rock or ledge you don't know about. You could use a post hole digger or steel rod to check that.
Have you amended the soil or mulched?
If you haven't mulched you need to recognize that avocado root system is shallow feeder roots which really need mulch to keep roots cool, reduce evaporation, maintain a soil food web and eventually build topsoil. If you pull mulch back under a healthy avocado tree you should be able to see fine feeder roots coming into the mulch. If not, get that started.
What is the sun and wind exposure?
If in full California sun or high wind exposure you might try a windbreak or something to give only afternoon sun protection but realize this tree needs enough sun to conduct photosynthesis.
What vegetation or trees are nearby?
I've seen lawn grass grown right up to avocado. The grass was absorbing almost all water and nutrients, the tree hardly grew and was starving. Same with a large tree, nearby trees can send roots out seeking water. Some can travel 20 feet and emerge into your avocado tree's root zone robbing it of everything. You could check by digging outside the avocado tree's root zone searching for small or large roots invading the tree's space.
How are you watering?
A two year old tree probably should have a root zone 3 feet wide maybe twice that if you have mulched well. As the tree grows you need to add mulch and expand the root zone by expanding mulching
outside the drip zone. Water needs will increase as the tree grows so a single dripper needs to change to 4-6 drippers or better a microsprayer which will maintain healthy soil throughout the root zone.
There's a slight chance some animal like mole or ground squirrel has a burrow or gopher has eaten roots?
Good luck.
PS, this guy's channel discusses lots of California specific tree information.
https://www.youtube.com/watch?v=fBMvGhHuXxU