WinXP will not Hibernate

One of my users Dell d620’s cannot Hibernate. I have tried installing the MS hotfix for memory changes (i.e. 1GB to 2GB), checking for error messages (there are none), checked for ACPI (yes) with hibernation support (yes), but the problem remains. When anyone tries to enter hibernation mode the screen flashes but never enters the hibernation screen and there is no sign of the process or any error messages of any kind.

Thank you for your help in advance.

– Space