Cutting peppers speeds up drying, but high humidity or cool weather can still cause mold. A dehydrator solves this. Excalibur is great but expensive. Try thrift stores for cheap Nescos.
I cut larger peppers in half and just take the tips off smaller ones. It makes them dry faster.
Use a dehydrator and make a single lengthwise cut on each pepper. They’ll be dry in less than 12 hours.
I leave peppers whole and dry them in the oven at 180°F for 6-10 hours.
I dry herbs and seaweed on a tray in the oven at very low heat. Works great.
I’ve strung them up plenty of times with no issues. No fans or windows needed. Let them hang until bone dry, then grind them. Grinding them can get rough on your eyes and nose though—be careful!
I use an oven or air fryer for 2 hours on low heat, then finish drying them in paper bags for a week. Grinding them into flakes works great.
I always use the oven to dry stuff.
If you’re stringing them up, make sure to run the thread through the green part of the chili. Avoid piercing the red part or they’ll mold.
I use a tray in the sun. Works better than stringing them up like meat. Sounds like you’re in a colder area—what’s a radiator? We don’t have those here.
Not enough air or too much humidity for proper drying.
Cut them open or put them in stronger sunlight.
Humidity is probably the issue.