Gwendoline - your mixture is probably too wet. Choc drops will drop to the bottom if the mixture isn't thick enough to hold them up. If you add milk, add less, or use smaller eggs, or more flour. (but not all those things at the same time - experiment).
Just made a banana and choc chip cake, which has bees in fan oven for nearly 45 mins at 180C and yet is nearly burnt on top yet still raw inside. Recipe says cook at 180C for 50 mins so should be nearly done. Have put it on middle shelf and have now covered with foil.
With my old fan oven I used to reduce temp by 20 degrees but if I do that with this new one it still overcooked top and doesn't cook inside. Oven is a whirlpool.