Explanation
Two potential reasons for SMS responses getting the default keyword response rather than the appropriate next keyword response are:
Responses are not sent within the Conversation Window. The Conversation Window is a setting that defines how long MobileConnect waits for a response from a subscriber after sending an outbound message. If a subscriber responds after the Conversation Window expires, MobileConnect treats their response as a new inbound message rather than part of an ongoing conversation.
Next keyword was not specified on the outbound message. The Next Keyword is a setting that defines what keyword MobileConnect expects from a subscriber after sending an outbound message. If a subscriber responds with a different keyword than what MobileConnect expects, MobileConnect treats their response as a new inbound message rather than part of an ongoing conversation.
Responses containing "stop" in the message content will not get the default keyword response, but rather an opt-out confirmation message. Responses sent with AMPscript in body will not affect how MobileConnect handles responses, as AMPscript isonly evaluated at send time. References:
https://help.salesforce.com/s/articleView?id=sf.mc_moc_conversation_window.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.mc_moc_next_keyword.htm&type=5