HUGE MISTAKE! Didn't correctly put this gut on a flush draw. Duh!

Wow, made one HUGE MISTAKE! I raised on the button to $15 with Ks9s. One caller. He "checked dark" and the flop was Xc-Xc-Xd. I bet $20 and he called. Turn he checked dark again and was a blank. I bet $30 and he called. River was a blank and he checked. I checked and he won showdown with KcJc. GEE, where did I go wrong??