They may be going through a mini molt, and not a hard molt. Mini molts are sometimes barely noticeable, whereas a hard molt is unmistakable. The first time you see a bird going through a hard molt you will say to yourself, "Ohhh... so that's what a hard molt looks like. Wow. That bird looks terrible."
I had a bird last summer drop all her feathers practically overnight. It was truly impressive. I had never seen it happen so dramatically before. The other birds in the flock of the same age all went through some kind of molt, but none so obvious and spectacular as her. She was completely bald except for about 5 feathers. Then she looked like a pin cushion for quite a while with all her new quills growing in. She went back to her beautiful self eventually, but I will never forget how ridiculous she looked for about a month.