Until this morning I assumed this due to the 4gb problem but i hadn't read and understood that problem properly.
When I upgraded to the latest beta firmware and my video library was still sparse as ever I realised that the missing ones all have a jpg thumbnail associated with each one. Conversely the ones that do display do not have a thumbnail file. All films list put fine, with or without thumbnail on my other DLNA players.
Apart from the obvious workaround of removing or renaming the jpg files, does anyone have any ideas? And what is the best way to report this problem to HUMAX?