Thanks for reporting the issue.
For each intent that requires fulfillment, you must enable fulfillment for the intent. Apparently you might have enabled fulfillment just for the Default Welcome Intent. Please make sure to enable it for all the required intents.
1. Click Intents in the left sidebar menu.
2. Click on the affected Intent.
3. Scroll down to the Fulfillment section and toggle Enable webhook call for this intent. [2]
For reference you can follow the steps on documentation [2]
[1] https://stackoverflow.com/questions/60503007/dialogflow-fulfillment-mutiple-intents