Frontier originally requested 4 slots at DCA, but were awarded two (one landing, one take off). Since then they have been after those additional two slots. Twice a day service makes better financial sense (overheads) than once a day.
Will they get them? Probably - eventually. They have proved they run a viable service, and there are a couple of "beyond perimeter" slots that could become vacant.
But right now, I don't think any new slots are being authorized - the concentration is on getting DCA back to "normal".