A properly browned scallop is one of the wonders of cookery, because getting it perfect isn't easy. First, scallops are crazy-wet, and moisture fights against browning. You can get a pocket of water trapped between the pan surface and the scallop, thus inhibiting the temperature rise needed for browning. Secondly, butter, while delicious, adds a higher degree of difficulty than vegetable oils, as its solids are prone to burning and its smoke point is fairly low. Master this and you'll be able to sear anything well.