what exactly are you guys saving ?
just one object ?

well...

there a many reasons to nHibernate not saving new instances. 

maybe are in 
try { } catch(Exception) { } 
and you cant see the error throws,

maybe (if u're using <many-to-one unique="true"> tag on hibernate, and
this object will be saved if u use property (cascade="save-update").

if in session.FlushMode is setted to FlushMode.Never
call a session.flush() before call commitTransaction (maybe it works!)


try change session.FlushMode options.


are to many reasons for it not work.
Post the code, and maybe some guy will help ya


good luck!