The conditions that existed for the one exist for the others, unless you change some factor(s). First, are you running a fan on your plants? If not, get some air circulation going, as it will help keep the surface dry. Second, are you bottom-watering, and ensuring the top does not get wet? And are you possibly watering too often (how often do you water)?
Look very closely at the surface of the peat pellets. For comparison, you might want to wet a new one. The surface of the ones with plants should still look very much like the surface of the new one. However, if they have a brown "mealy" look, they probably have the fungus. Scrape it off as carefully as you can, then start misting the surface with a mix of hydrogen peroxide and water. Also, look carefully at the base of the rest of the plants at the soil line - if any look mushy, soft, wet, they're probably starting to succumb to it. Sometimes they can be saved by staking them up and using the hydrogen peroxide/water mix, but no guarantees.