Second monitor on motherboard not detected

I have been using two monitors with one connected to my graphics card and another connected to the motherboard via the VGA output. I recently tried to connect a third monitor to the motherboard DVI-D port using a DVI-VGA adapter but the monitor is not detected and does not receive a signal from the PC. I am using the most up-to-date drivers from the Chipset and VGA driver sections of the ASUS website. The only relevant BIOS setting is iGPU Multi Monitor which is already set to ENABLED in order to use the second monitor (via the VGA port). I've swapped the monitors around so I know the monitors are both working.

I have something called Intel HD Graphics Control Panel installed and it lists only the single monitor.

OS: Win 10 Pro 64-bit

Mainboard: ASUS Z170 Pro Gaming, board revision 1.04, BIOS v1904

CPU: Intel i7 6700, Stock HSF, Undervolt Offset -0.15 V, LLC 4

RAM: Corsair LPX Vengeance 2x8GB DDR4-3000 (CMK16GX4M2B3000C15)

Graphics: Gigabyte NVIDIA GeForce GTX 950 w/ 2 GB

Sys Drive: Samsung Evo 750 SSD, 250 GB

Data Drives: OCZ Vertex 2 SSD, 60 GB; WDC WD1002FAEX-00Y9A0, ~1 TB; WDC WD2003FZEX-00Z4SA0, ~2 TB; WDC WD2002FAEX-00MJRA0, ~2 TB; RAID 0, ~300 GB [Maxtor 6B160MO x 2]

PCI-E SATA expansion: IOCREST SATA III 4-port PCI-e (SI-PEX40064)

PSU: XFX TS 750W (P1-750S-NLB9)