I had this problem the other day: my macbook pro was not sleeping when closing the lid.
I was 100% sure I have not modified any power settings nor installed new applications so I had to dig up a bit more.
Reading few websites about similar problems got me to a tool which helped me find the source of this problem.

The command pmset -g was showing me several applications that could prevent my system from going to sleep (line in red).

florians-MacBook-Pro:~ florian$ pmset -g
System-wide power settings:
Currently in use:
 womp                 0
 halfdim              1
 sms                  1
 hibernatefile        /var/vm/sleepimage
 gpuswitch            2
 networkoversleep     0
 disksleep            10
 sleep                0 ([color=red]sleep prevented by UserEventAgent, UserEventAgent, configd, backupd, backupd, mds_stores, AddressBookSourceSync[/color])
 hibernatemode        3
 ttyskeepawake        1
 displaysleep         15
 acwake               0
 lidwake              1

However, it did not provided more details about them.
The proper command to get these details is pmset -g assertions:

florians-MacBook-Pro:~ florian$ pmset -g assertions
2017-01-14 01:15:38 +0200 
Assertion status system-wide:
   BackgroundTask                 1
   ApplePushServiceTask           0
   UserIsActive                   1
   PreventUserIdleDisplaySleep    0
   PreventSystemSleep             1
   ExternalMedia                  0
   PreventUserIdleSystemSleep     1
   NetworkClientActive            0
Listed by owning process:
   pid 63(UserEventAgent): [0x00001b41000b852c] 00:06:53 BackgroundTask named: "com.apple.mds.0" 
	Created for PID: 87. 
   pid 63(UserEventAgent): [0x00001776000b84b5] 00:23:04 BackgroundTask named: "com.apple.backupd-auto" 
	Created for PID: 257. 
   pid 74(configd): [0x0000161a00078478] 00:28:53 DenySystemSleep named: "InternetSharingPreferencePlugin" 
   pid 2151(backupd): [0x00001796000184bf] 00:22:32 PreventUserIdleSystemSleep named: "Time Machine backup" 
   pid 2151(backupd): [0x00001796000b84be] 00:22:32 BackgroundTask named: "Time Machine backup" 
   pid 2732(AddressBookSourceSync): [0x00001cdb00018554] 00:00:04 PreventUserIdleSystemSleep named: "Address Book Source Sync" 
	Timeout will fire in 1796 secs Action=TimeoutActionTurnOff
   pid 330(mds_stores): [0x000019d1000b850b] 00:13:01 BackgroundTask named: "com.apple.metadata.mds_stores.power" 
   pid 131(hidd): [0x00000f81000982b3] 00:57:02 UserIsActive named: "com.apple.iohideventsystem.queue.tickle.4294968500.11" 
	Timeout will fire in 895 secs Action=TimeoutActionRelease
Kernel Assertions: 0x108=BT-HID,MAGICWAKE
   id=503  level=255 0x8=BT-HID mod=01/01/1970, 02:00 description=com.apple.driver.IOBluetoothHIDDriver owner=BNBMouseDevice
   id=504  level=255 0x100=MAGICWAKE mod=01/01/1970, 02:00 description=en1 owner=en1
Idle sleep preventers: IODisplayWrangler

By looking at the details I found one process that I recently enabled: Internet Sharing.
That was it! Once I disabled the internet sharing, my macbook was able to sleep again!

If you are in the same situation, I hope these two commands above will help you find the way.
Cheers!