How can I figure out why Suspend sometimes fails?

Something is causing suspend crash occasionally. When it does crash, the system freezes with a black screen and stays on, not suspending. This makes me have to hold the power button until the system shuts down.

One setting difference I have from most people is, I don't use the option that lets you suspend when the laptop lid is closed. So opening and closing the laptop lid has no actions. I like to press suspend manually. Could this preference change be the cause?

How can I start looking for what's causing the crash, since the crash doesn't display errors?

I don't think the setting you mention is related.

Trouble-shooting

Error messages relating to suspending are typically found in /var/log/pm-suspend.log so have a look there.

Typical cause

These problems are usually due to some process(es) stopping the system from being suspended.

Do

dmesg -T|grep Freez -A4

and look for these entries:

--
[sun mar  3 15:19:48 2013] Freezing user space processes ... 
[sun mar  3 15:20:08 2013] Freezing of tasks failed after 20.01 seconds (3 tasks refusing to freeze, wq_busy=0):
[sun mar  3 15:20:08 2013] mount.nfs       D e8631aa0     0  5518   5517 0x00800004
[sun mar  3 15:20:08 2013]  e8631b10 00000086 f7bc0e00 e8631aa0 c1053cb4 c1809020 c192ee00 c192ee00
--

Check the time stamps to see which of the reported problems relate to your try to suspend. In this case, it is mount.nfs that is causing the problems.

Suggested workaround

Have a script run automatically before suspending and have the script kill the offending process:

The user can provide scripts in the /etc/pm/sleep.d/ directory. Those scripts will be run by the system at suspend and resume. The file name should start with an ordering number. 00-49 is used for user scripts. For details, see man pm-suspend.

The script could look like this:

#!/bin/sh
(killall -9 mount.nfs; exit 0)

...with correpsonding entries for other processes that caused problems, if any.

Parenthesis and exit 0 is a trick: if the process isn't found, killall will exit with en error exit code which will cancel the entire suspend. The parenthesis will make killall run in a sub-shell which will exit with exit code 0, regardless of the killall exit code.

If you're having problems, check /var/log/pm-suspend.log since it will also log problems relating to running the scripts in /etc/pm/sleep.d/.

No, disabling the "suspend on lid close" function cannot affect manual suspend.

You would start by looking at the output of dmesg and the contents of /var/log/kern.log and /var/log/syslog for clues.

We can try to help if you share these with us. See this question for how:

How can I easily share the output of a command or a text file with others?