Hello,
From how you describe the problem, I believe that the 'arret' in the winding spring has broken of (mind you, I'm certainly not an expert)
To prevent the winding spring to snap when winding, there's a system in place where the end of the winding spring 'hooks' to the inside of the spring barrel.
If the spring is wound to far, the hook will slip and 'recatch' (after one turn), preventing the spring to break.
I think the 'hook' broke off, causing the spring to rotate freely, without being caught/held at the inside of the barrel.
So you can keep winding it endlesly and the power reserve will not be built up to full.

Hope this helps, just my 2 cents...

Kind regards,
Bart

P.S.: If this is the case, you will need a new winding spring, a watchmaker that can order it/has one in stock, and who knows how to install it...
GOOD LUCK!