Trouble with getting data from compute shader

Hello everyone!
I try to develop compute shader for Marching Cubes algorithm.
I setup all using Temaran guide GitHub - Temaran/UE4ShaderPluginDemo: A tutorial project that shows how to implement HLSL Pixel and Compute shaders in UE4

I successfully setup shader but have the problem data exchange between shader and C++ program.

I upload around 700 Mb voxel data to shader but when I try to get more 31 Kb data from shader I get only zeros. When I get less data, all fine.

I actually need to take more 10 Mb data back. How can I achieve this?

Data is stored in AppendStructuredBuffer and I copy it with this code:

    srcPtr = (uint8*)RHILockStructuredBuffer(shaderOutDataBuffer, 0, sizeof(FOutTriangle) * total_size, EResourceLockMode::RLM_ReadOnly);
    // Reference pointer to first element for our destination ComputedColors
    uint8* dstPtr = (uint8*)_meshData.GetData();
    // Copy from GPU to main memory
    FMemory::Memcpy(dstPtr, srcPtr, sizeof(FOutTriangle) * total_size);
    //Unlock texture

If total_size less than 450, all works fine but I have much more triangles in out.

doesnt this happen on the cpu ??