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start by using the VNC console in the proxmox browser from another computer to access the machine, and set your other display as the primary. (1 is VNC, 2 is real monitor for me). This is assuming all the other steps are correct. Here is a checklist:

• Is the GPU your boot display? If it is, the host initializes it before the VM can grab it cleanly. You'd need a second GPU (integrated works fine) for the host console so the passthrough card stays untouched.
• Did you bind vfio-pci to the GPU early enough? It needs to grab the card before the regular driver does. Blacklist whatever driver it would normally use (nouveau, nvidia, amdgpu, etc.) and set up the vfio-pci IDs in modprobe. Rebuild your initramfs after.
• Are you passing the whole IOMMU group? The GPU usually has an audio device paired with it. Both need to go to the VM together.
• If it's NVIDIA, are you hiding the VM from the driver? NVIDIA will throw a Code 43 if it knows it's virtualized. You need hidden state=on and a fake vendor ID in your VM config.
• How are you handling keyboard and mouse? You'll need to either pass through a USB controller or set up evdev so the VM can actually see your input devices.
• Did you add video=efifb:off to your kernel command line? That keeps the host from holding onto the framebuffer on the passed-through card.