Since a short while I get this error.
Tried to extent the $conf['update_max_fetch_time'] = 30; and even to 60 but does not help.
Also with drush pm-update does not work, set there drush vset update_max_fetch_time 60 but no luck always just a partial update list.
Deleted the cache but still the same sometimes it gets a few projects sometimes almost none.
I also have some sites on other servers and exact the same problem. It does not seem to be related with the servers or the sites since it has been working correctly and just recently these problems started. Some sites are not upgraded and on lower versions and never had this problem but since shortly they do have the same problem.
The problem occurs with 6.x as well with 7.x sites (have no 5.x sites) on 3 different servers. So it looks like it is drupal site related.