In fact it went down at one point from D- to F with blue potential. I have been practicing 21 minutes at FT's for quite some time and it won't go back up from F. Customer support says it isn't broke..It really seems that way to me.

https://www.whatifsports.com/hd/PlayerProfile/Ratings.aspx?tid=0&pid=3138612

I don't know about Free Throws (if there is a cut off) .. but there is for other attributes.  If you are below this cutoff, it does not matter if you practice or not, there is no growth.

One way to get into this is to have a Red Attribute that you put 0 into .. that attribute drops.  If it drops to the point where it turns black, then you put in points, no growth.

There is also a point, below 10 I think (could be wrong about that, it might be lower), where there is no growth regardless of color.

So, there may also be a point in Free Throw Shooting where no matter what you do, no growth .. if there is, I have never actually seen it.

Ask them why its not going up when you put 21 minutes into practice time?

From my experience free throw is slow growth, where you notice the improvement is in the off season.