Were all three trips open then covered at the same time?
The "if needed" YS is only looked at when the pilot has been identified to fly (i.e., you're getting SOME trip), there are one or more additional pilots also identified to fly, and there's multiple trips to be covered.
If (for example) the junior pilot got tagged to fly his trip before you, THEN the JFK trip hit OT and was covered as the only trip in that bucket... then you get it, and the "if needed" doesn't come into play. In other words, your YS can't force the senior guy to fly if he's not already tagged to fly one of multiple rotations. On the other hand, if the JFK was the only trip to be covered when it got sent out, it would go to the junior pilot (since your YS was "if needed"--and, with only one trip to cover, you weren't needed). Then the next trip comes out, you're first in line, and TAG!, you're it....