Blender detects the GPU, I have GPU compute selected and it runs in the rendered viewport, but when I tell it to render, it only uses the cpu and not the GPU.
Like, nothing runs in my system. Fanns are silent, GPU shows no activity, nothing.
But then, sometimes, randomly, it starts working and rendering. Like, what’s going on?
GPU shows no problems running in Resolve or even while playing video games. It’s only with Blender. Why?
Tell us something about what CPU and GPU you have, and what version of blender you’re using.
If rendering settings are normal, rendering itself will not be done if GPU rendering is not processed. 
The CPU is also used in the rendering process of GPU settings.
If the rendering setting is set to GPU, the rendering processing works as GPU.
It may be because the rendering process requires less time or less processing.
My CPU: AMD Ryzen 7 5700X
My GPU: RTX 3060ti
Blender 4.3
I have a scene that it should render in 37 seconds using the GPU, but because the gpu doesn’t start up with Blender, it takes 10 min instead.
However, I found out that If I open Blender set to Optix, the gpu does not start up at all, no activity, the fans don’t turn on, nothing. But if I change to CUDA, then it works. And now here is the funny part.
If I change it back to Optix, it now renders with Optix.
This only happens with Blender. Substance Painter, Davinci Resolve and videogames use the GPU without issue.
I was thinking maybe it same something to do with the kernels?
Please uninstall and reinstall the GPU driver completely. 
I’ve been having several GPU issues lately, and I think I probably need to uninstall and reinstall the controller as well.
Many of the problems with GPUs in blender are related to drivers.
My personal opinion is that GPUs aren’t as stable as CPUs. There’s a lot of influence from drivers
Which ideally should be a fairly recent release, only the WHQL ones and unless you happen to be one that likely to play the latest AAA games on release day, then just use the Studio Drivers.