My Brooks have been ridden in torrential downpours and on long hot sweaty rides and have not sagged significantly. You may, for whatever reason, need to tighten the adjusting screw in the nose of the saddle. Brooks makes a special wrench for this adjustment that is intended to take out sag. It's unusual to need to do this after only 2 weeks, but it sounds like it's worth a try. Just don't get too ambitious about it or you can ruin the saddle.