Comment 5 for bug 136433

The excerpt from the changelog quoted above explains it:

powermanagement-interface (0.3.16) gutsy; urgency=low

  * pmi.acpi: Disable suspend and hibernate if /host is mounted via fuse;
    disable hibernate if a swap file is active.

 -- Colin Watson <email address hidden> Wed, 29 Aug 2007 18:40:49 +0100

Try

$ grep ' /host fuse' /proc/mounts

If there is an entry matching that, the changes shown above will cause suspend and hibernate to be disabled.