Hi
Not sure what could be causing it - session data is stored by default in a common area on the server - so it could be some kind of (space???) problem there.
I have done a
website hosting where the session data is stored in a database rather than files (in my case this was to get round the 30,min restriction on the persistence of the data) - this would probably be a good long term solution for you (not much use in the immediate situation I know, but worth considering longer term).
Odd that some data is saved and not other bits.
Sorry I'm not being not much help - but it does sound an odd one