Strange ws2811 led issue

Hi I really hope someone can help with this strange issue with some ws2811 pixel leds. I have some set up as you can see in the diagram, I'm not sure if this makes any difference but they are controlled by a remote control (sorry cant post link, it doesnt work for some reason) but if you search tesfish 5V-24V RF Remote Controller on amazon it's the first one that comes up.

Here is the problem, the first 6 pixels work fine, the 7th pixel flickers slightly with subsequent pixels flickering more.
Now the distance between the controller and the 1st pixel is less than 1 meter. The first 5 pixels have about 0.5M between them and the distance between pixel 5 & 6 is about 3.5 meters. I know this is relevant, the cable between pixel 5 & 6 goes underground ( the black shaded area in the diagram ).
The reason I know this is relevant is because if I wire pixels 5 & 6 with a wire above ground which is the same length as the underground wire pixel 7 onwards dont flicker even slightly.
So I'm slighty confused as to why having the cable above ground makes a difference, BTW I have pulled the underground wire out and examined it, it has not been damaged in any way or kinked.

What I also dont understand is why pixel 6 works ok but the others are starting to flicker, I would have thought if the signal is getting distorted by going underground then pixel 6 would be flickering as well but obviously this is not the case.

Any help would be really appreciated, thanks.

With 12V ws2811 led strips, you must cut the strip only where indictated, which is after every group of 3 leds. You cannot cut it between leds 5 and 6. There are no pads there to make connections to. You would have to scrape back the backing strip to access the copper traces, and you would find there are 5 or 6, not 3.

PaulRB:
With 12V ws2811 led strips, you must cut the strip only where indictated, which is after every group of 3 leds. You cannot cut it between leds 5 and 6. There are no pads there to make connections to. You would have to scrape back the backing strip to access the copper traces, and you would find there are 5 or 6, not 3.

No it's not strips I'm using, it's pixel string, I have cut the wire and soldered wire between them to space the leds out more.

Ah, I see, sorry. All the ws2811 pixel strings I have come across have been 5V, so when you said 12V, I wrongly assumed it was a strip. Posting a link to the spec of the components you are using is recommended in the forum guide, helps avoid such misunderstandings.

I would recommend, for any longer lengths of interconnecting cable, to put a 330~510R resistor into the data line, close to the data input of the next led in the string. Also a capacitor, 100~470uF, across the 12V & GND connections, close to the same led. Check the cap is rated over 12V.

If that does not help, try replacing the suspect leds. The fault could also be in the output signal of a led that appears to be working.

Thanks ill give that a try.