If the display is not connected directly but through an HDMI extender, scaler, matrix switcher, or similar AV device (e.g. Extron, Crestron), a modeline generated with {{ic|cvt}} or {{ic|gtf}} may be accepted by {{ic|xrandr}} (the output reports the new resolution as active) while the screen itself shows no picture. This happens because such devices are built around the fixed [https://en.wikipedia.org/wiki/CEA-861 CEA-861/CTA-861] timing table used by HDMI/TV equipment, and their receiver chips may refuse to sync to a VESA CVT-generated timing even at the identical resolution and refresh rate, since CVT and CEA-861 define different blanking intervals (and therefore a different pixel clock) for the same active resolution. This can happen even when the device's own EDID does not advertise the resolution at all: some devices' EDID under-reports what their receiver hardware can actually lock onto.
Compare the device's own advertised modes in xrandr --verbose: if its listed timings use round CEA-standard pixel clocks (e.g. 74.250MHz for 720p60, 27.000MHz for 480p60), the device is CEA-861-oriented and a CVT/GTF modeline for a higher resolution is likely to be rejected even though the same resolution works fine on a directly-connected computer monitor.
If a CVT-based custom mode results in no signal, try the standard CEA-861 detailed timing for that resolution instead of one computed by cvt. For example, 1920x1080@60Hz per CEA-861/CTA-861 (VIC 16) uses a 148.5MHz pixel clock, distinct from the 173.00MHz (standard CVT) or 138.50MHz (CVT reduced blanking, {{ic|cvt -r}}) that {{ic|cvt}} would generate for the same resolution:
$ xrandr --newmode "1920x1080_CEA" 148.50 1920 2008 2052 2200 1080 1084 1089 1125 +hsync +vsync
$ xrandr --addmode HDMI-1 1920x1080_CEA
$ xrandr --output HDMI-1 --mode 1920x1080_CEA
Unlike cvt, there is no command-line tool included with X.Org to generate CEA-861 timings; they are a fixed table rather than a formula, so the values must come from the CEA-861/CTA-861 specification itself (or be read from the detailed timing descriptor of a directly-connected HDMI display's own EDID, which will already encode them, e.g. via edid-decode or xrandr --verbose).