Why is my white sapote's new growth failing? See pics

[FigoVelo]

Hi everyone. My largest white sapote tree is struggling to put out new growth, and I can't understand why. For at least a month it has been pushing buds from the tip of its main stem (which last year grew about three feet). But continuously, the new growth seems to dry up, fizzle out, fail - however you want to describe it. I have checked the tree at night and have seen no earwigs (though last year they were all over this tree). I also have seen no slugs on the tree. Can anyone guess what is happening to these new little buds and why they won't grow beyond this stage? Notice in the pics how the ends of the tiny leaves are blackened. We have not had a hard frost here for about 6 weeks.

Thanks,
Alastair

[JR561]

My white sapotes had tons of issues dealing with full sun.

Once I gave them some shade they took off.

I was getting discouraged the past year.

[Flgarden]

My white sapote redlands was struggling with sun while in a pot. This year i planted it in ground, it was not looking good on the sun for a few months but started growing like crazy now.
Ana

[Owen H]

I was having the same issue after mine defoliated due to freeze over the winter. I put 50% shade cloth over it and it seems to be coming back now

[FigoVelo]

I’m starting to suspect I have over fertilized. I suppose all the growth last year plus a small crop of fruit got me excited, and a little greedy.

What is the best way to address this problem? Do I need to heavily prune the tree? Or just flush out the soil? Any specific flushing strategies?

Thanks.