Oof, this got long, sorry about that.
Man, I first found you when someone posted TROS's demo in the otome thread on the ChoiceofGames forum, and I've downloaded and loved every game you've published. Hearing that Heaven's Grave isn't doing well is incredibly disheartening. I teared up multiple times while playing---something that I rarely do---and thought it had some of the strongest, most complex writing you've done so far.
IMO, it's not the price of HG that is affecting sales---$15-20 USD is fairly standard for indie otome I've seen and a godsend when you can't afford $30-60 commercial games---nor is it the art, which may not be commercial-game quality but is very clean and pretty, or the story since fantasy otome always seem to be popular. Like another commenter has said, it's most likely the promotion of the game. I don't recall seeing your games promoted (much) by the otome blogs/twitters I follow, and it might have taken longer for me to find you if not for the COG forum. I definitely think that for future games, you should try to increase your social media presence and post frequency and then ask prominent otome/VN blogs to reblog/follow you.
Moving on to BFF, I've never supported games on Kickstarter, even if it's a game I'm excited for or a creator I really like, usually because the predicted release of the game is too far away for me to feel confident that they'll deliver and/or it's their first big game. However, you pump out quality games fairly quickly for such a small studio and keep in constant communication with players, providing regular updates and replying to all comments. Because of that, I have a lot of faith and trust in you as a fan and would be perfectly comfortable supporting BFF on Kickstarter, even though I'm much more of a fantasy/historical-fiction kind of person. For tier rewards, if you don't want physical items, you could do things like backer-exclusive DLC (which I would be happy to pay for), custom messages from a chosen love interest, backer-exclusive art, custom written scenarios chosen by the backer, access to the beta, digital art books with concept sketches and dev notes... And not sure if this would be possible, but maybe you could offer artbooks/rewards from your previous games? I, for one, would love to hear more about the worlds of TROS and HG.
Going back to when I mentioned ChoiceofGames, I don't know if you've explored the forum at all but have you considered using Choicescript to tell your stories? As a CYOA, you wouldn't need art (but can include some if wanted), music, or GUI, although it would need more writing to make up for the lack of visuals and the most I've seen a Choicescript game charge for is $5-7 USD for a 150,000-300,000 word game, which COG's publishing company Hosted Games takes a cut of. But COG does provide a forum for users to post their WIPs and get exposure and feedback, and also will promote the games they receive for publishing. I've seen people who've turned their VNs into CYOA and vice-versa, but it's not the right format for everyone; I just wanted to recommend you look into it if you ever need to.
In any case, thank you for your hard work and I hope everything works out well for you and BFF, I'll be supporting you no matter what!