Problem setting new CV120 value for a 4-8-2 Light Mountain

[mikew2]

I own two Bachmann N scale 4-8-2 locos (Missouri Pacific 5203, Southern 1489). I've had the MP since early in 2019; the Southern was acquired Feb '20. Since both had the same default whistle, I wanted to change the whistles for each loco. There was no problem doing this for the Southern, but attempts to set different values for the MP's CV120 ended with a 'No Decoder ACK pulse' message on my Digitrax DCS 52. What could be causing my two locos to be acting differently?