Please see image Last Ac)tion
Some remain at Process and never make it to print or preview. Any known reason for this?
Could it be that you have some customizations on the forms that are failing? Sometimes a bad customization can run for a longtime and not output a proper error message.
Other than that, I think we need a bit more info. Are the failures all from one PC? Do the failures happen consistently to a specific form? Which ones? Are there any customizations on those forms?