After a solid month (game time) I have not been able to increase fp with any villagers that are engaged (I have seen their heart events). I'm not engaged either but wanted to have max fp before going that far. Have I missed an event somewhere that is preventing any increase in fp?
I can't seem to raise fp enough to get the household sunstone which means I can't raise mystic island and get these folks married and possibly increasing fp that way.
I think you just haven't been trying to raise the FP long enough. If you've only been talking to them and not giving them gifts, it will take you at least two months to gain one heart, so I recommend gifting and using the wild animal trick, as well as talking to them, viewing heart events and winning festivals. Here's the link to the Sunshine Islands page on Friendship.
I may have misunderstood your question, but I think this is what you meant.
Make raising Mystic Island a priority. I did not raise Mystic Island in my first time playing and had a glitch where no one had more hearts until I raised Mystic Island. I gave all the bachelors and bachelorettes their favorite gifts for a month and there was no advancement.
Once Mystic is raised you will also have to prepare for multiple weddings. Any couple will get married who has been engaged for more than one week will get married on consecutive days until all are married. This means getting your cattle fed and overwatering your crops.
This is what happened to me. There is another thread where I advised raising Mystic Island and it was corrected. If the bachelors and bachelorettes are not high enough in friendship, this could be why you are not getting the head of household sunstones.
I think the friendship points should start to rise after you raise the island.