PCS Question

So I put in for a swap with the pot for a trip. It is a 5 day trip running the same days as the trip I want to swap it for. The only difference is better layovers and higher value.

I look this morning and I am not awarded the trip. Someone else junior to me is awarded as a white slip. The trip does not depart for a week.

I thought PCS was suppose to honor seniority. Can someone explain this one to me.