Why don't you always get lust from flirting?
All chat options have a limit to the amount they can boost a stat. The limit is actually quite low aside from dating options. For flirting, you can fill up the first heart, and a small bit of the second. Furthermore, some girls can be pushed a bit higher than others, I assume this is because of opinions, but not really sure. Regardless, the difference is minimal.
The other possibility is that you're charisma stat is too low. I don't know about flirting, but with low charisma, small talk occasionally doesn't allow you to select an opinion, resulting in no love increase (which also has a limit). I assume that flirting works similarly to small talk.