What command base are you driving them from? If you're driving them from a Legacy or BASE1L, it doesn't have the same serial drive as the old TMCC BASE1. In the case of the newer products, you'll need a serial port booster.
One solution to boosting the serial data is to use the Lionel 6-81326 LCS Serial Converter 2 (SER2), it provides higher serial signal drive to solve this issue. You'll also need the Lionel 6-81499 LCS Power Supply with DB9 Cable.