Render Texture seems to go blank when sent to Compute Shader?

I have a render texture (that's being blitted to from a webcamtexture, if thats helpful), that works fine when I blit it in turn onto the screen - but when I send it to the compute shader, write whats on it onto a different texture, and blit that new texture to the screen it's entirely black.

Relevant CPU code :

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 [RequireComponent(typeof(Camera_Handler))]
 public class Gol_Handler_Camera : MonoBehaviour
 {
     public ComputeShader compute;
     public int _width;
     public int _height;
 
     int width_groups;
     int height_groups;
 
     RenderTexture _Input;
     RenderTexture _Output;
     RenderTexture _Screen;
     RenderTexture _Video;
     RenderTexture _Video_Save;
 
     void OnRenderImage(RenderTexture src, RenderTexture dest) {
       Graphics.Blit(_Screen, dest);
     }
 
     void Start() {
       _Video = GetComponent<Camera_Handler>().Camera_Render();
       _Video_Save = GetComponent<Camera_Handler>().Camera_Render();
       _width = _Video.width;
       _height = _Video.height;
 
       _Screen = new RenderTexture(_width, _height, 0);
       _Screen.enableRandomWrite = true;
       _Screen.filterMode = FilterMode.Point;
       _Screen.wrapMode = TextureWrapMode.Clamp;
       _Screen.Create();
 
       compute.SetInt(width_id, _width);
       compute.SetInt(height_id, _height);
       compute.SetInt(buffer_length_id, _buffer_length);
   
       compute.SetTexture(4, input_id, _Input);
       compute.SetTexture(4, output_id, _Output);
       compute.SetTexture(4, screen_id, _Screen);
       compute.SetTexture(4, video_id, _Video);
       compute.SetTexture(4, video_save_id, _Video_Save);
       compute.SetBuffer(4, buffer_id, _Buffer);
 
       width_groups = Mathf.CeilToInt((float)_width/8f);
       height_groups = Mathf.CeilToInt((float)_height/8f);
     }
 
     void Update() {
       compute.SetTexture(4, video_id, _Video);
       compute.Dispatch(4, width_groups, height_groups, 1);
     }
 }

Relevant GPU code:

 #pragma kernel Initialise
 
 uint _width;
 uint _height;
 
 RWTexture2D<float4> _Video;
 RWTexture2D<float4> _Video_Save;
 RWTexture2D<float4> _Input;
 RWTexture2D<float4> _Output;
 RWTexture2D<float4> _Screen;
 
 StructuredBuffer<uint2> _Buffer;
 uint _buffer_length;
 
 ...
 
 #pragma kernel GOL
 
 ...

 #pragma kernel Display

 ...
 
 #pragma kernel Reset
 
 ...
 
 #pragma kernel Video_Input
 
 [numthreads(8,8,1)]
 void Video_Input (uint3 id : SV_DispatchThreadID)
 {
   _Screen[id.xy] = _Video[id.xy];
 }

If I change the OnRenderImage blit from blitting _Screen to blitting _Video it works fine, so clearly the _Video render texture contains data prior to being sent over to the shader - but once it's there it seems to behave like it's completely blank. I ideally want to be able to alter and use the data from the _Video texture once its in the compute shader, not just send it back to the screen, but I can't even seem to do that at the moment.

Any help would be really appreciated.