Either the letter hasn't been written, or it hasn't been mailed, or the postal service lost it, or it wasn't mailed with the matching form, or the dentist did everything right and AADSAS lost the letter/matching form, or the LOR is sitting is a pile at AADSAS and waiting to be filed. If you call AADSAS they'll tell you what you already know and nothing else because they access a screen similar to what you see when you log onto AADSAS. You might want to ask the dentist if the LOR has yet been mailed.