In order for a model to be activated, all of it's mandatory triggers must be activated.
In the screenshot you provided, Add's X trigger will not be activated, because the source of the flow into it, namely the Fleet CP field of API Econ is empty, since API Econ has no other incoming flow, therefore it does not exist.
If you change X to a non-mandatory trigger (colored yellow rather than green), by right-clicking it and un-checking Mandatory, Add will work, with the <Sum> being the value provided in Y (received from Game States), but I'm pretty sure this (namely all API Econ fields being empty except for Fleet CP) is not what you intended, especially given the fact that you're trying to Update (rather than Insert).
I note that in the screenshot you provided there's an additional trigger (above the one providing Game States), so perhaps you provided an incomplete screenshot here, and that trigger is actually used to provide the previous API Econ state, even so it is entirely possible that the Fleet CP field is empty (NULL in database parlance, which is completely different from 0), and therefore Add's X trigger should still be non-mandatory, to be able to handle this case as well as the general case where Fleet CP has been previously set.
In addition, it is recommended that you avoid "loops" (I refer to the fact that both API Econ and Add are source and target of each other), making the order of flow more explicit clearer.
Lastly, you should make it a habit to encapsulate chunks of your model to make sure they run in the correct order, which results in the following recommendation:
I Hope this clears up a few of the issues you're trying to understand and resolve.
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