Dynamic scheduling vs resource capacity

[Dusty Rhoades]

I have a project schedule which I'm attempting to forecast with the help of Portfolio. However, I'm running into a slight roadblock while using the tool. I have up to three different scenario based plans to help find the team's balance between scope bloat, resource capacity and item estimate/weight.

Initially each of my scenarios had the same total developer(s), total items, item estimates and release parameters (dynamic scheduling) - which resulted in the same release date being scheduled. However, once modifying the resources by scaling their 40hr weekly capacities down to 30, 20 and eventually 0. Portfolio holds to the initial date it had dynamically scheduled. And when adding to the original estimates per item within the stack Portfolio begins flagging random items and notes them as 'cannot be scheduled'.

Shouldn't Portfolio instead tell me that additional sprints are required within my schedule and forecast those items based on estimates and my resource capacity?

My goal is to baseline my ideal target release date then layer in a 'buffer zone' of scope and time based on the amount of bugs typically received during our sprint verification process.

Any advice?