Something that should be added to a list in an if statment is not being added

I am trying to loop through a list of objects, It happily loops through most of the objects, and works for the majority of them, however a couple of times (and always for the same ones) it will not work. I am trying to add the object to the list if its ID does not match he Teacher ID of a previous Lesson in that time period (SubBlock/SB), the one particular ID that it doesn't seem to like is 'ECA', who is free to tech all the lessons, out of the 6 i am trying to get it to pick ECA (the old viable option)it works on attempt 1,3,5 but not 2,4,6, The function it is working through is below

function getRelaventFreeTeachers(&$PossibleTeachers, $Teachers, $Lessons, $SB){
    foreach ($Teachers as $TeacherID){
    $Usability = 1;
    foreach ($Lessons as $Lesson){
        $LT = $Lesson->getLessonTeacher();
        $T = $TeacherID->getTID();
        $LSB = $Lesson->getBlock();
        if ($LT != $T){
            if ($LSB != $SB){
                $Usability = 1;
            }
        }else{
            $Usability = 0;
        }
     }
     if ($Usability == 1){
         array_push($PossibleTeachers, $TeacherID);
     }
   }
}