New HP Envy 16 inch Ultra 7 does not communicate to Samsung 27" 4k monitor

[pppppppp]

I bought a new HP Envy '2 in 1' laptop Ultra 7. When I connect via HDMI to my Samsung 27" 4k external display, that display remains dark. The Envy Display options indicate awareness of an attached second display yet when I ask it to identify the external display, a number 1 shows on the Envy screen, but the external display remains dark.  If I move the HDMI to my 2017(?) HP Spectre X360, the external screen immediately illuminates and is recognised as an external display.  I would be very grateful for any ideas

[banhien]

@pppppppp 

What is your HP Envy '2 in 1' laptop Ultra 7 machine ?  Please use the following way to find out:

                 How to Find Your HP Laptop Model Number

Regards.

[pppppppp · Accepted solution]

Thanks Banhien - after switching cables a number of times, the external monitor started to flicker. Initially I changed the display setting on the HP Envy to 50Hz and it worked, but subsequently stopped working; I changed it to 25Hz and it seems stable. 

Thanks for taking the interest but I think the issue is resolved now