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Find out what is preventing your Mac from going into sleep mode by running the pmset -g assertions command. In the example below, the PreventUserIdleSystemSleep is set to 1, which means the system is being kept from going into sleep mode. The com.apple.ppp.pptp and com.apple.ppp.l2tp are the process that are preventing the system from sleeping. Disabling the VPN server allowed the system to sleep.

echelon:~ dervish$ pmset -g assertions
10/20/13 8:32:17 AM MST
Assertion status system-wide:
PreventUserIdleDisplaySleep 0
PreventSystemSleep 0
PreventUserIdleSystemSleep 1
InternalPreventDisplaySleep 0
ExternalMedia 1
DisableLowPowerBatteryWarnings 0
UserIsActive 0
ApplePushServiceTask 0
BackgroundTask 0

Listed by owning process:
pid 36(powerd): [0x0000000900000249] 17:13:31 ExternalMedia named: “com.apple.powermanagement.externalmediamounted”
pid 67(com.apple.serve): [0x0000000100000137] 25:56:48 PreventUserIdleSystemSleep named: “com.apple.ppp.pptp”
pid 67(com.apple.serve): [0x0000000100000136] 25:56:48 PreventUserIdleSystemSleep named: “com.apple.ppp.l2tp”
pid 8690(httpd): [0x00000001000006dc] 11:54:12 PreventUserIdleSystemSleep named: “com.apple.apache.denysystemsleep”