MacBook doesn't always enter sleep when lid closed

Recently my MacBook 15" Early 2011 has developed a sleep issue. Very often, when the lid is closed, the fans keep on running and the MacBook runs warm. The LED-indicater on the front turns on. The machine will remain in this state, so no sleep happens. Then when the lid is opened, the LED-indicator turns off, but the screen doesn't light up. Closing and re-opening the lid doesn't change a thing. The only remedy is forcing a power-off.

Things that changed recently:

• I replaced the harddrive. I installed a Samsung SSD 850 EVO 500 GB (firmware: EMT01B6Q).
• I enabled TRIM (sudo trimforce enable).
• I reinstalled OS X 10.11.3 and restored a Time Machine backup.

Things I've already done:

• Reset PRAM and SMC.
• Verified energy saving settings.
• No peripherals attached (power adapter only).
• Updated to OS X 10.11.4.
• Check the console for messages. No seemingly interesting messages were posted before closing the lid, and no messages are written until after forcing power-off. So no messages are written for when the lid is opened.

Other information:

• 2 GHz Intel Core i7
• FileVault Disk Encryption
• No optical drive installed
• 2x 4 GB memory (non OEM)
• The MacBook is sometimes slow to wake
• Putting the system to sleep before closing the lid has the same issue