When travelling through reeds with your River Raid Longship, Eivor constantly orders the mast to be pulled up and down - Eivor would say "mast up!" and "bring the mast down!" 2-4 times in quick succession and after that, you can no longer raise the sail so you're forced to row the ship for the rest of the raid.
If you look closely at the mast in the video, you can see that it pops up and down and up and down as if the longship constantly goes in and out of stealth
- this is also indicated by the white shimmer that occurs every time you're in stealth, the shimmer flashes in and out. From 1 minute onwards in the video, I am constantly trying to raise the sail but nothing happens.