Watery stools after switching formula

Hi Mummies, my lo is 6 months+ and recently we have transit his formula from Brand A stage 1 to Brand B stage 2. Ratio is 50:50 and we noticed that lo has watery stool. May I know does it mean that the new formula is not suitable for him? Or 50:50 is too much for him to take it? Or I can't change from Stage 1 to Stage 2 directly if it is different brand? The reason for changing is because the previous formula is causing him phlegm.