If you read the details on the Data Roll Over feature, you will see that it will only roll over unused data in your base plan, and that roll over is only good for one month. that means if you used all your base plan data in any month, you have no data left in it to roll over. If you didn't use it all, then your previous roll over data "expires" and the new data you have remaining from your base plan will roll over into the following month. This same process will happen every month. If you use all of your base data in your plan in any month, there is nothing of that left to roll over into the next month.
I would guess that you used all your base plan data last month, so you had nothing left of it to roll over into this month.