Italian prune plums not ripening
Thank you for the image.
The small yellow fruits which aren't developing weren't pollinated. That may occur for various reasons at bloom time, including rain, low temperatures, and/or lack of bee activity.
Even if only half the fruits set, it's wise to thin the fruit so that branches don't break when the fruits are mature. Thinning fruit (removing the excess) promotes fruit size and quality. To do so, remove the excess by creating 6-inch "fruit-free zones" between adjacent fruits.
See "Fruit thinning" -- https://extension.oregonstate.edu/gardening/berries-fruit/fruit-thinning