It sounds like you're applying the motion blur to an individual layer. While combustion will let you do this, the number of samples can only be determined by the original footage, so greater samples won't appear to do anything. If you're trying to get motion blur on the movement of the layer, you'll need to nest the layer first, then apply the motion blur operator. Then the samples can work on the subpixel movement of that layer, not just the frames of the layer.