[Pelican]

With all deference to those who have obviously put way more thought into this (I mean, I learned things just reading the posts above), I can suggest some admittedly basic, maybe dumbed-down, "quick fixes".

I set batter and pitcher development speed to at least 1.15, basically because I'm impatient. That does seem to move things along, both good and bad results. (I also set aging to 1.15, which prevents guys from hanging around too long. It would though tend to prevent the Verlander/Scherzer kind of recent seasons, if you like that.)

I focus on college and juco players in scouting and the draft. There is just too much wild variation in prep guys. They are too hard to project, even with the very best scouts. Good SEC players are about ready to play in MLB after a partial season in the high minors and maybe some Winter ball. If I could access the wood bat Summer league stats, I could predict performance with more confidence.

I use high accuracy in scouting, and I put lots of resources into the scouting department, and hire highly-rated scouts with experience. (My ideal would be a combination of Clint Eastwood and Amy Adams in "Trouble With the Curve".) I understand and do not judge those who prefer the "fog of war" approach. I am not a fan of surprises. The more information, the better.

I also use a now "tried and true" approach with Player AI Evaluation settings of 40% Ratings, 30% current stats, 20% last year stats, 10% previous year stats. That seems to work for me, maybe because I have used it for years in all leagues, and I am accustomed to how it works. Actually, my advice on this critical setting would be to "set and forget" - choose (there are many threads on this choice, and as many options as there are posters, so you can't really go wrong...) one formula and then stick with it, to reduce variability in player evaluation.

I have found, to focus on the original OP dilemma, that it can be true in OOTP that "the night is darkest just before the dawn". Those prospects who show little or no improvement for several seasons, stagnating ratings, can still turn it around, all at once. And in fact this same thing does happen IRL, particularly with the best prospects. (It's a topic for another thread, but updated scouting evaluations can cause a prospect's Potential ratings to go up - and of course down - significantly. So never give up because Current ratings are static and Potential has been reduced. What goes down can still come up.)

I'm a Phillies fan, so I tend to think in terms of Mickey Moniak, a seemingly wasted #1 pick, who eventually developed with the Angels, and had a breakthrough year of sorts last season with the dreadful Rockies. But for every Moniak there will be a Scott Kingery, can't-miss guy who actually had a very solid year in 2019, got a big contract, fell off the map. Right now the Phils have SP Andrew Painter, still highly-rated by some, but a disaster last year at AAA coming back from surgery. Frustrating. And OOTP does a good job IMHO of duplicating that frustration. If all this were predictable, it would be boring.

When all else seems to fail, I like the approach of promoting an underachiever up to MLB (obviously in a context where the team can afford it; not in the midst of a pennant race). There are mysteriously some guys who suck in the minors and excel in the majors, Spencer Strider and Max Freid, for example. (Why are they always Braves?!). My guess is that some guys just cannot stand the minor leagues, but are ready to go when the MLB bell rings. Fine. No way of knowing that unless you try.

Injuries will happen, and that of course tends to blow up development. OOTP is good at this, in terms of frequency (I have it set to low, because in my OOTP world I don't want to lose guys and thus not get to enjoy seeing them play) and the rehab process. I doubt that Painter will ever be a star, and have traded him in several seasons (the AI loves him, and the return has been excellent!). But the Phillies may be right to be patient, and he may pop in that first full year recovering from major arm surgery.

The posts above suggest this may be more science than art, if you know how to deal with it. For me, it seems to be more art (luck) than science, no matter how carefully I nurture the young guys. That of course is what makes it so very cool, when (if) a guy you patiently brought along blooms on the MLB level.