There could be a few factors.
One: repotting right away when you bought it might have shocked it- especially when it's in a new environment.
Two: I'm assuming it was purchased as an indoor plant so direct outdoor sunlight would also cause damage- especially if it was already shocked from the reporting
Three: If you feel you have been watering but the bottom is really dry then sometimes it could mean you are watering inconsistently. It's tricky- you can't water too much, but also not too little. I always feel the soil about once a week for indoor plants before I water. Drainage is your friend, some rocks at the bottom of your pot or well drained soil mix will help a lot! Also clip the dead leaves off right away so your plant can focus on new growth