It normally takes about a week for tall fescue to germinate. So, I
wouldn't be too worried yet, and certainly wouldn't seed again now.
It's possible that the areas that have not come up yet are cooler,
maybe less sun, or more wind causing cooling through water evaporation,
which would slow germination. Were those areas treated with any
herbicides other than Roundup? With Roundup, its recommended to wait a
minimum of a week before re-seeding, which I hope you did.

If you don't see growth in another week, then I'd take up a square foot
of soil and carefully start investigating to see if there is any sign
of germination. If there isn't then I would seed it again. You'd be
near the end of the seeding window at that point, but should still have
enough time for it to take.