I was running A DC-6B to KATL with 100% LF. I then opened a second route with an electra, forgot to account for the speed difference and ended up with the return flights 5 minutes apart. Ever since then, the my DC-6B hasn't carried a single pax other than when the electra flight canceled while the Electra has ~99% LF. I understand that pax don't like flights right next to each other and prefer faster A/c, but it seems a bit extreme.The Electra also has 17 less seats than the DC-6B, which was constantly filling up, but not even 1 pax will fly my DC-6.