While running an extended water quality model on a large system, several of the water tanks in the system seem not to interact with the rest of the system correctly. For example, one tank shows no flow in or out of the tank for the entire duration of the model (15 days). Consequently, the volume of water in the tank never changes, and the water age increases with time at a 1:1 rate. Another tank shows flow in and out for portions of the simulation, but experiences regular periods with no flow whatsoever.

Any ideas on what might be preventing the tanks from interacting with the system for extended periods?