The conditions may not be as similar as you think they are. Plus, the smaller ones may have more underground competition than the others. Plus those on the ends are more exposed than the others.

Available root space? Mature ficus fill the soil with roots until no space is left. What about adding new soil around the stumps.

How big were the holes you dug when you planted them? Possibly the holes were not large enough. Another factor is that some plants are just stronger than others, even when planted in identical conditions.

thanks for the ideas ... is it worth it to get a soil test kit and check to see if there is any difference in these areas that are only a few steps apart?