hi.i had loft converted into two bedrooms and bath room three years ago.plaster boards are used on partition and solid walls .after 6 months some cracks appeared on ceilings. two places where ceiling and partition walls meet cracks are bigger.i have filled bigger cracks twice with filler but filler cracks again.what could it be.i know builder have used scrim on ceilings but not on wall and ceiling joints.whats solution.what type of filler can be used so it does not crack.any advice is welcome.thanks.