When you open a list do you see a "clear" button near the upper right corner? This would indicate you have a filter enabled. By tapping on the "clear" button the filter will be removed and you should see your tasks.

When you open a list do you see a "clear" button near the upper right corner? This would indicate you have a filter enabled. By tapping on the "clear" button the filter will be removed and you should see your tasks.