Charcoal grill not getting hot enough
It really sounds as if you're just getting the tail-end of the fuel left in the charcoal.. that you've let it burn too far in the chimney .. 40 minutes does sound like a long time .. the right time to start cooking on charcoal is when any self-sustained flames have gone down, and the surface appears grey-white in daylight.. if you fan it, there should be a clearly perceptible glow.

If you want to use your chimney, give the charcoal just enough time in there to propagate its own burn.. don't spread it too thin in the grill after transfer, and let it settle to a glow in the grill, not the chimney. That will also help the metalof the grill heat up.

Or, light the charcoal in the grill by fanning it gently, and later, not so gently, with whatever comes to hand, or an electric fan.