player progression
 

Do player's ratings improbe as the season progresses, only in the offseason, is there any tricks to enhancing player development?

Player's ratings constantly fluctuate. I think the coordinators and playing time are two of the biggest factors in successful player development. Although, there are many people on here who know much more about this than myself.

Player's ratings (the red bar) usually improve the more the player plays. There are instances where it might not, and there are instances where even potential (green bar) increases in time, but I think that requires good coaching.

There also seems to be a certain hidden factor in the game that makes player ratings (both red and green bars) fluctuate in the offseason.

Other significant factors include coaching and injuries. You also need to consider the scouting factor. Ratings can fluctuate based on your scout. And when you sign a free agent, his ratings may change after you sign him. I think this is because the ratings you see whn he is a FA are based on "average" scouting, and the ratings you see after signing him are based on your scout's perception.

also........if one player shows a 19 out of 65, and one shows a 21 out of 35, does that mean that it's better to play the second for now, but the first has more potential?

I would play the 19/65 guy because the playing time is going to increase his current ratings and it won't be long beforehe goes past the other guy's 35 future potential. By playing the 21/35 guy you are slowing the development of your (potentially) more talented player.

another question for the experts.........is there a way to improve player endurance?

Although I am no expert.. I believe that endurance is a stagnant rating. :(