i believe this is what happens when u change brands in generic. the concentration of the active ingredient may be less due to poor manufacturing process..At the time u were on the previous one that worked.... the amount of DHT u blocked was just sufficient to ive u result but when changing to a different brand which have lower concentration, yr DHT levels spiked and yr hair system recognised a change that now the dht is slightly higher and start shedding hairs.
This is jsut my guess.
This is jsut my guess.