This might have to do with your not sending the entire command. For example there may be a command:
"VOLT:RANGESET 2.0" after that it's possible to send either "VOLT:GAINSET 2.0" or "GAINSET 2.0"
The instrument command parser many times maintains a relative path of sorts. I'm not sure about the commands you're using but you had VSET and ISET, it might be worthwhile to send the entire command string for ISET.