I find this a bit confusing and I think I have been doing it wrong Whey do they stack and when don't they stack?

Replacement effects do not stack (Raise the Stakes: unopposed damaged is raised to 2). Additive effects stack (Across the Anoat Sector: SS units gain a blast damage).

Could you post a specific example? It might be easier to help you then.

In general, though, you want to pay attention to the wording. For example, a hypothetical card that sets your reserve value to 7 would not stack because no matter how many copies of that objective you have in play, it still sets the value to 7. If, however, you have multiple objectives that increase your reserve value by 1, those would stack incrementally.

I think I get it now. Thanks. I think what happened I read a rule about something not stacking and made it universal. ugh..

In general, things stack in this game. If something doesn't stack, it's because the wording just makes effects redundant, not because they don't stack. Using the example of Raise the Stakes that doctormungmung mentioned, it's not so much that the effects don't stack as it is that having 2 different effects that raise unopposed to 2 results in unopposed being 2.