It sounds like you have an ill-defined problem with an algebraic loop. Specifically, A_Total is a function of a, but a is a function of A_Total. To address this problem, you need to make a modeling decision about how these quantites should be determined, and then update the rules accordingly.
For example, yould could decide that A_Total should be set to a fixed value, and delete (or inactivate) the assignment rule that was setting A_Total.
