I recently reported this issue to google (like so many others did, no
doubt) and this was the reply I got....... *cut 'n paste*

Thank you for your note. The group you mentioned,
rec.aquaria.freshwater.plants,
was recently the target of a spam attack. The spammer posted a large
number of
messages to several groups and then removed them. This caused the
appearance of
gaps in the archive of the group's posts. Please be assured that no
content
posts were removed.

Regards,
The Google Team ........


This doesn't really seem to shed any light on why the older posts still
aren't appearing though .