Try another yarn that matches the weight–length ratio (yardage per ball) given in the pattern. Swatch again. ❓
Try another yarn with a matching weight–length ratio so it’s closer to the pattern’s recommended thickness. Swatch again. ❓
Go a needle size down and swatch again, trying to knit with a slightly tighter tension. ❓
Go a needle size up and knit a new swatch. ❓
Proceed to check the row gauge.