So if a player has low potential in a certain category and you don't put any practice time into that area and the rating drops - if the player still has low potential can you later put practice time into that cat and will it at least get back to the previous level, or if the player has low potential in that area, is that lost forever?

Should come back easily.

your question is a strange one. i think you have a misunderstanding about what potential means, so i will give you the quick rundown.

potential as displayed to us is merely a derivative. it is NOT a core attribute of a player.

really, a player has 2 values for each rating - a current value, and a maximum value. for example, a player might have 50 speed, with a max of 60 speed, and thus, we are told he has medium potential.

so, in your question, the guy having low potential just means the different of his max and current value is 5 or less. if you end up losing a bunch of points, his potential will be higher - the potential is just the difference of the max and current. so for example (although outrageous, just to illustrate), if you have a guy with 80 speed low potential, and you don't practice and he drops to 60 speed, then he would have high potential.

I think that the way it is described is the way that it is supposed to work -- but I don't think it in fact works that way.  

For example, on several occasions I've had players with low potential in stamina lose 4 or 5 points in stamina in the offseason - then with 20 min or more in stamina not regain a single point.

stamina is different. For some unknown reason HD code writers felt that every human should lose stamina when they're not attending regular classes during the fall or spring semesters. If a player is near their max in potential or has a low work ethic, they may ezily go backwards in the summer each season and never make it back up... No earthly explaination will/can explain why but just know it can happen. Stamina is in a category of it's own. Don't tell your kids to stay in shape during the summer... because HD made everyone of their fake basketball players summer-time loafers in gaining stamina... it simply can't be improved once the sacred gym doors are closed for the summer. Their all gonna get drunk and come back fat and lazy... some may have massive improvement in nearly every concievable area.... but the will have LESS stamina.

I seem to recall someone had a ticket with CS re stamina drops with low potential and determined there was a problem.  That somehow the improvement rate is not adjusted when stamina falls, making it more difficult to regain lost points when at low potential.  Maybe someone can find the thread.