So..
Sufficient water in the reservoir to keep the pump happy (so it doesn't try and push air around instead)?
Pump is running and vibrating quietly?
Water is going round?
Bubbles slowly being pushed out of your loop and collecting in your Reservoir?
Any kinks or sharp bends?
Can you see any large bubbles causing an airlock in your loop?
Got kitchen roll handy for mopping up?
Proving your loop is working can be a slow process, but patience is key.
My first loop I rushed it, and smoked a gfx card and a motherboard.
Sufficient water in the reservoir to keep the pump happy (so it doesn't try and push air around instead)?
Pump is running and vibrating quietly?
Water is going round?
Bubbles slowly being pushed out of your loop and collecting in your Reservoir?
Any kinks or sharp bends?
Can you see any large bubbles causing an airlock in your loop?
Got kitchen roll handy for mopping up?
Proving your loop is working can be a slow process, but patience is key.
My first loop I rushed it, and smoked a gfx card and a motherboard.

Years ago now anyhow. I'm on my third watercooled build now, and all three are still going strong.