Your frequency is probably too high. 3 flights per day on a 100 seater would be better....
This is really some kind of magic. it's very hard to guess in advance what kind of frequency you can afford on a specific line. I did just put 2 daily CRJ100 flights on a 250 demand line, with opposition(2 difference companies with a 130-seater each), and I'll see how it behaves, but my uneducated guess is that it will behave as if there was no opposition. If I put a third one, I'll be at the limit of frequency, I guess...but I'm not sure.
I also really felt the difference on a 800 demand very short route with a lot of opposition. Going from 8 to 12 regional jets(old F28 in that case) saw my LF down from 60% to 40%(with RI far from 100). Even with 100%RI now, my LF is rather crappy. I guess this third plane with 4 more flights was not a good idea. It did behave as if there was no opposition, and a much smaller demand.
I'm still learning this, as it's my first real game with big demands everywhere. I'm used to small routes, where one single daily flight is the maximum, and opposition is scarce. It's nearly another game. I'm not yet fully efficient in this new game.