I wandered around a medium-sized swamp for 2 nights, when the moon was on the Waxing Gibbous and Full Moon phases, and I found 0 slimes during the entirety of that time. Is it just a 1.8 glitch? I have been needing slimes for a long time now, and I'm frustrated that I have found none during the time they should be spawning the most. And no, it is not on difficulty "Peaceful".
Slimes only can spawn in a swamp if the swamp is between Y-levels 50 and 70. In addition, slimes will also only spawn if the light level is at most 7.
To be more accurate:
"More precisely, the game checks if the light level is equal to or less than a random integer (from 0 to 7), then if the fraction of the moon which is bright is greater than a random number (from 0 to 1). If these conditions are met and the altitude is acceptable, there is a 50% chance of spawning a slime."
Slimes can actually be rather rare if the random number generator does not like you.
In addition, slimes follow all normal mob spawning rules. They will only spawn if you are within 16 to 144 blocks from a "valid" area.