The most important factor is the type of scallops you are using; dry scallops or wet scallops. Wet scallops, as their name suggests, are soaked in water and usually chemically treated to preserve them longer. This means they absorb and retain more water which will hinder the browning process of the scallops.
Dry scallops are the ones you want to use to achieve a nice brown crust. Dry scallops are not chemically treated and are not soaked in water. Their flavor is therefore more pure and concentrated but their shelf-life is shorter so eat them fast.
Another important step in cooking scallops and other seafood is to not wash or rinse the scallops. Thirty minutes prior to cooking just place them on a paper towel lined plate and cover with another paper towel on top. If they are dirty or smell funny and you feel they need to be washed then you should return them and not eat them.
Finally, it is important to have a very hot pan and to not move your scallops until they have browned. Moving them during searing will remove them from the direct contact with the heat and will mean they will be less likely to brown or will take longer to brown.