Why won't 1 out of 3 monitors go to sleep?

Question

Monitors

Three monitors are connected to the system unit, one of the monitors does not enter sleep mode, why?
Available:
- Motherboard: GigaByte H87N-WIFI rev.1
- Processor Intel i7-4770S
- OS Windows 7 64bit
- Monitors 3pcs - Samsung 27A350H (monitor settings are identical)
All monitors are connected to the motherboard, there is no external video card.
The motherboard has 2 HDMI connectors and 1 DVI.
2 Monitors are connected via HDMI, and the third one is DVI-VGA i.e. analog input.
The first monitor (Windows calls it 1st), which is connected via HDMI - does not enter sleep mode, there are no problems with other monitors.
If I change 1 and 2 monitors in places, then the problem is again with the one who stands first.
Despite the fact that the monitor does not enter sleep mode, the image disappears and a window appears - no signal. But it does not go into sleep mode. He tries to sleep, but immediately wakes up with this sign that there is no signal.
PS. And monitors come out of sleep mode quite normally.

Answer 1

I solved the problem, after almost a year:
I set the video output priority to IGFX, but I had to select "PCIe 1 slot", I note that I do not have an external video card.