Hello, and welcome to the site.
It sounds to me, like its your EQ [Erection quality] to blame, rather then a loss of size.
This is because, your flaccid stretched length [FSL] is your maximum bone pressed erect length [BPEL] and is used to track gains before they reach permanency, because you see it in the FSL before the BPEL.
Its possible you didn't measure 100% correctly when you got 21cm, or, if you did PE at the time, the gains [growth] wasnt cemented, and made permanent.
This is due to FSL always being the size or your maximum erect length.
So, if you have lost anysize, it would have been temporary from penis enlargement, or, if you didn't do PE at the time, you won't have lost size, and what you see now is down to either or both, an error in past measurements, which is common, always use the same point to measure from i.e. use Bone pressed against he pubicbone from the same angle with the same ruler if you can.
And, a change in your EQ which is affects size. Things like illness, stress, colds can all affect EQ.
I think its a mixture of both in varying degrees, and the best course of action is from today start a fresh, with a new measurement using FSL with a ruler you will always use, on the same spot, same angle, same position either sat or stood, but do the same eachtime.
Check the newbie routine, which is powerful even though its called newbie.
Ask any questions you have, and we can help you.
I know that, clearing the slate and starting up fresh is always a positive way forwards, and, imho this is what you need here.
Don't look back at past measurements after today, have your current FSL, forget the erect stuff for awhile, lets concentrate on the newbie routine, the length only part of you like, and in several months time your FSL will have changed, which translates into a larger erect size.
