Gary-
One of the things that popped up in the last version was a lack of player development in long term sims. The same players tend to dominate until they age out with no younger players gaining on them. It is still happening in this version from what I've found.
I've done a few tests, but in the most recent I created two guys with a good potential for growth. You'll see in the screenshot (sorted by potential growth), the top three players with in potential growth are 1) my created golfer and 2,3) golfers I edited to have additional potential. The golfers I gave more room for growth, did improve pretty significantly. The game seems to only generate small growth potentials, though (21 max), so those guys are not able to compete with established players.
Image didn't seem to work. Link...https://imgur.com/a/JmxWm2i
It looks like players CAN develop, I just think you need to have more players with a higher potential.