I installed Linux Mint Mate 15 x64 (based on Ubuntu 13.04) choosing the "encrypt home dir" option.
So my swap is encrypted as well. (configured automatically by the installation)

Challenge
Sometimes when I try resume the laptop's screen doesn't turn on. Resume from suspend only works 95% of the time.
Moving the mouse and pressing any buttons (incl function keys) does nothing. I have to do a hard power off.

===========================================================

Investigating Resume Settings
(I have obfuscated my UUID's below, using a different letter for each unique UUID)
[0] % cat /etc/initramfs-tools/conf.d/resume
RESUME=UUID=A

[0] % sudo blkid
/dev/mapper/cryptswap1: UUID="B" TYPE="swap"

as you can see the UUID in my resume file does not match my cryptswap1 UUID. (neither does it match any of my volumes listed by blkid)

I ran the command "ls -l /dev/disk/by-uuid" and the UUID specified by the resume doesn't match any of my disks.
I'm unable to get the blkid (using the blkid command) for the partition that the cryptswap1 partition is on, while the cryptswap is mounted.

[0] % cat /etc/fstab
#UUID=C none swap sw 0 0
/dev/mapper/cryptswap1 none swap sw 0 0

Okay, now I'm really confused. None of these UUIDs match.
===========================================================

My swap also disappeared recently on one occasion.
After a fresh boot (or when suspend/resume works properly), "free -m" reports many GB of free swap space.
I have suspended and resumed a few times today without problems. My swap is still intact. (shown by free -m)

Suspend works by putting the OS state in RAM, not in swap, while hibernate uses swap. Sometimes the word suspend is used for either.

Which are you actually using?

I am definitely not hibernating.
Mint Mate x64 has two distinct options "Suspend" and "Hibernate".
When I said suspend, I mean "Suspend".

---------
Out of interest...
Hibernation is not even possible with a standard installation where the swap is encrypted.
The reason for that is the swap volume is mounted with a randomized encryption key on every boot.
There is a hack to type in a password on boot to use the same swap encryption key every time, but I've not tried that yet, and so hibernation is not available to me.

Then in that case all the info you provided about resume settings wouldn't affect the problem. Like I said, suspend works by using RAM, so it's that that you need to investigate.

How does manually activating suspend work for you? I mean by actually typing

echo -n mem > /sys/power/state

in a console. Is it more successful?