Why does my gathering party not gather the amount it should..?

If you are a raider, you have a permanent debuff on gathering. For instance, you need a total troopload of 7.3 million to carry 3.7 million lumber/food.

Iron takes up double the space in troopload. So, if your legion troopload is 7.3 million, your legion will carry only 1.8 million iron.

As my legions are equipped with golden medals and lanterns (they gather extra resources), I fill put at least the first row with T1 cavalry (highest troopload) and add other combinations to make the reported troopload of 7.3 million or more. Only then my legions can finish a level 9 resource plot.

Cavalry gear boot and breastplate increase the troopload of legions. When I did not have enough t1 cav, I put cavalry breastplace on iron gathering legions and cavalry boot on lumber gathering legions.

I hope that helps.