I've got firmware version 1.63 and have started getting this problem. A new folder I created would not appear to DLNA clients but would appear for anything using SNB (or whatever it's called - when you look on a PC or using a file explorer on my phone).
After messing with media server settings I gave up. Then the next day, the folder was there!
Then I went to it again today and it had gone from DLNA again.
I plugged a USB drive in which seemed to force a re-scan of everything. I took the drive out and after a delay, I can now see the new folder on DLNA again! I'll have to see if it sticks this time.
The date isn't an issue on my system - it's set to use NTP and it's working well.