Molds on bark medium........Why??

[Pinoychids]

Hi all, I recently repotted all my orchids in clay pots with barks medium about 3 weeks ago. All orchids are placed on wet gravels or stones. Today, I noticed that 1 inch down the medium there were molds everywhere. My seedllings' roots are all rotting. One mature plant had rotted root, which I saw and pulled through the side drainage hole. What causes this problem? lack of room circulation? Too much humidity? Any suggestions to solve this problem?

Thanks all in advance.......