I have similar problem. Seems only a few posts of the uploaded posterous backup xml file is imported.
My Posterous export xml file "wordpress_export_1.xml" is too big, > 25M, so I tried to split it at the boundary between </item></item> tags into two parts 01.xml and 02.xml with similar size around 12.5M (since the WordPress.com posterous importer dialog told me that max size is 15M).
After splitting the file, I appended the "</channel></rss>" end tags to 01.xml and prepended the xml header tags by copying from 01.xml to 02.xml, so both files should be complete in format.
Since the Posterous export file is without \n line ending but a single line, it's hard to edit in text editors, so I pre-processed them with Perl to add \n to tags with space or tab characters next to them, with the substitution statement "s/>[ \t]/>\n/g".
I tried to upload the resulting 01.xml file to wordpress.com through the Posterous importer menu. It told me that authors are not found, I got an email reporting successful import, but there is no posts imported even after 24 hours.
Then I tried to search and replace all the authors in the 01.xml file to be the same as the WordPress blog to be imported into and tried again, this time the author is recognized and I selected the mapping to current author. After that, the same result is found, none of the posts were imported.
Then I tried to convert the xml file back to be one big line by stripping all the \n characters with a Perl script to "chomp" each line. I uploaded the resulting 01a.xml file and this time I got 16 posts imported, while the xml file includes over 600 posts. Moreover, all the titles of the 16 posts are blank, which is also not correct.
I guess maybe the number of posts are too big, so I reduced the number of posts included in the xml file to only 10 posts by manual editing the xml file. After uploading it, to my surprise, only 5 of the 10 posts were imported. I checked the titles of the posts and found that only the odd numbered (1, 3, 5, 7, 9) posts are imported.
I'm going to try importing the 10 posts one by one, to see what happens.
How to solve this problem? Since Posterous.com is closing the end of this month, I'm facing an urgent problem.