Check out Zenon filters. They are now owned by GE. They are nanofilters (0.02 microns). Auto cleaning. Expensive. If it is a particle; this will fix it. It takes out viruses.
Depending on what the geology is like you may want to look into hydrofracting the well. If you are mostly in rock, it could help. I don't know enough details for other geology. It uses high pressure water to fracture the surrounding rock. My well went from losing a contest with my kidneys and a six pack to about 3.5 GPM.