More than once I had found myself perplexed by a fret that would not gracefully seat itself completely in a fret slot.
More often than not the problem was the slot being too shallow for the tang on the fretwire. I saw the slots to an appropriate depth when making a dulcimer fingerboard but by the time the fingerboard is trued and leveled the slots sometimes become too shallow.
After having this happen a few times I came up with a very simple tool to solve the problem.
I took a piece of fretwire and filed the barbs off the tang so it will easily fit into a fret slot. if the slot is deep enough the crown of the fret will seat well on the fingerboard. If not then I need to deepen the slots.
The tape on the end of the fretwire it to remind me that this is a tool and not a stray fret that escaped the blow of a hammer.