The evaporation-rate slider controls the evaporation rate of the chemical. The diffusion-rate slider controls the diffusion rate of the chemical. The ant colony generally exploits the food source in order, starting with the food closest to the nest, and finishing with the food most distant from the nest. It is more difficult for the ants to form a stable trail to the more distant food, since the chemical has more time to evaporate and diffuse before being reinforced. Once the colony finishes collecting the closest food, the chemical trail to that food naturally disappears, freeing up ants to help collect the other food sources.
Real ants use a variety of different approaches to find their way back to the nest.
Try to implement some alternative strategies.