First of all what end stats are you looking at if you are looking at the RF stats then you should have blocking trend on cell level as well. Please see the hourly trend on these cells since SDs are utalilzed for MTC,MOC Location Update, SMS MT and SMS MO. along with other reasons then you can see the SMS usage distribution as well to narrow out the cause. Usually such problems occur at the Location Boundary. and there are some parameters you can play with. Please check what is the major cause of SD utalization on those sites.