The most likely explanation damp-off from being too long under the humidity dome. And yes it can easily affect some and not others.

This is a common problem often discussed over on the Growing from Seed forum here. As soon as the first few seedlings begin to break the soil surface and you see a hint of green stem showing the lid needs to come off or at the very least propped open. If you wait for all or even most of them to come up or for most of them to be standing upright then it is too late. Once the first few show, the rest will follow even without the dome on.

Keep in mind that humidity domes are not required for germination. They are just a "tool" and they often create more problems so many don't use them at all. So no you definitely didn't want them under longer.

A set of T8 bulbs puts our minimal heat and the light intensity isn't too much for them so no the light wasn't the problem.

The next most likely cause is over-watering and root rot of some of the plants and yes, it too can affect some but not all.

Hope this helps.

Dave

Damping-off was my first thought as well.

Peat pots are not a dream come true for any seedlings, and peppers in particular won't like them compared to seed starting mix or potting soil.

In my experience, peppers like to be kept on the dry side until they have four or five leaves. They germinate better for me if the seed starting mix is lightly moist rather than wet. I start my seeds in mid March so the seedlings won't have to hang around, waiting for the soil to warm up.

Thank you for your replies. Sorry for the delayed response. Although some of the leaves on a few of the pepper plants are curled, most of them look pretty healthy and are even growing new leaves. We are very hopeful that they will survive! We went ahead and planted another tray just incase though, and will not make the same mistake with the dome. Thank you again for your replies. :)