I've had a lot of variance in growth and fruit set on my 12 trees planted in nearly identical locations. Some are 12' tall, some are 4' tall, and they're all the same age. They're pretty tolerant of different growing conditions. Mature trees are huge, so yours might just be a little young to establish a regular fruiting cycle. I haven't seen any research done on rootstocks and growth, but variance in genetics is always a possibility.
https://journals.ashs.org/hortsci/downloadpdf/journals/hortsci/42/6/article-p1329.xml
https://webarchive.library.unt.edu/eot2008/20090117152821/http://www.hort.purdue.edu/newcrop/morton/white_sapote.html
https://apps.cals.arizona.edu/arboretum/taxon.aspx?id=439
https://gardenoracle.com/images/casimiroa-edulis.html
Reedo, good links to casimiroa, I have found growing w, sapote can be interesting for sure as you mentioned different growth rates with same age trees. I've grown w, sapotes since i was part of CRFG group that went down to Bob Chambers grooves to help save his varieties by harvesting as many varieties that we could find off his maps. This was in 2011 and I still have varieties as single trees that are only 3-4' tall and other multi-grafted varieties that are trying to be huge. I had found that one variety rootstock is vary vigorous and that is Pike variety there are others too but I think if you want a vigorous tree try planting a vigorous seed from robust variety. Also, experimenting with casimiroa tetrameria (yellow wholly leaf) as rootstock for dwarfing white sapote when grafted onto it. My friend Jack Swords first found this out my making a dwarf tree that way, also trying white sapote rootstock with yellow grafted on to it then graft white back on to it and see what size tree you get. Many folks now live in smaller yards and need smaller trees.
By the way that group that went down to Bob Chambers place was started with help from Bob Holzinger and Jack Swords, Harvey Correia, John Valenezula, and myself helped harvest scion wood from conflicting maps.
Very cool, thanks for the info! I was on the board of Monterey CRFG for a couple years and struggled to find any white sapote material locally. Jonah (Bush2Beach) was really the only person I could find with labels on his trees actively sharing scions. If you have any cultivars worthy of preserving, I'm active with the MBCRFG group, and am currently the only one bringing white sapote scions to events. I'd be happy to graft up and distribute anything I don't already have that's a favorite of yours. I currently have Vernon, Pike, Nettie, Lemon Gold, Wilson, Cuccio, Santa Cruz, Walton, Malibu #3, Michele, Delta Gold (Harvey's selection from seed), Lammertz, and Nova (a Bay Area selection from Laurie Mintz that is reported to have coconut notes). Only a few varieties have fruited for me so far, but they're definitely a family favorite!