Most likely your Tasks are turned off for this Contact. You need to ensure they are enabled to receive Tasks. To turn them on:
1. Navigate to the Contact in your Contact list and tap their name to go to the Checklist screen.
2. You should automatically see the Checklist screen, but if not, just tap the tab at the top to view.
3. Scroll to the bottom and check if the toggle is green.
If it is gray your will not receive any Tasks for this Contact.
Simply tap it to start receiving Tasks for this Contact. You will see the toggle turn green.