Some program is preventing Windows 8 from hibernating

Question

I have a Windows 8 laptop. Hibernate usually works fine, but occasionally it doesn't work.

When it's playing up, I click "hibernate", and the screen goes black, but then a few moments later the lock screen appears and the machine doesn't hibernate.

I find that if I close enough programs and try again, hibernate starts working again.

1. What is going on here?
2. If some specific program is preventing hibernation, can I identify which one? Can I stop it?

Update with more data:

I thought I might have found a specific program that's causing this, but now I'm not sure. I wonder if I have too much data resident in memory or something like that.

The event log appears to have the following error whenever a hibernate is aborted in this way:

Log Name:      System
Source:        volmgr
Date:          ***
Event ID:      45
Task Category: None
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      ***
Description:
The system could not sucessfully load the crash dump driver.
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="volmgr" />
    <EventID Qualifiers="49156">45</EventID>
    <Level>2</Level>
    <Task>0</Task>
    <Keywords>0x80000000000000</Keywords>
    <TimeCreated SystemTime="****" />
    <EventRecordID>109087</EventRecordID>
    <Channel>System</Channel>
    <Computer>****</Computer>
    <Security />
  </System>
  <EventData>
    <Data>\Device\HarddiskVolume4</Data>
    <Binary>0000000001000000000000002D0004C003200000170000C000000000000000000000000000000000</Binary>
  </EventData>
</Event>