Your linear spread for a resource assignment is only showing for a part of the activity duration.

If you are seeing your spread for only a portion of the activity, you may need to check the assignment. Check to see that you did select "linear" for the curve. If you have to select it again, do so, then schedule with the most recent data date and then summarize again. After summarizing, check the data in the resource assignments section. There are some cases where you will also need to check the resource assignment start/finish dates in the event that you added the activity, progressed it, added resources, and scheduled etc.