how to tap on a .fbx file(including a group of objects) without adding 'box collider' component?

Hello, everyone. I have a question in doing the gesturework in unity. I import a .fbx file which is a floor into unity, and the floor including a group of small objects. I have drag,tap,rotate gestures on the floor, then I add component 'box collider' in floor. My problem is when i tap on the floor, i can't not drag it. However,if i don't add component 'box collider' in floor, then the drag function works and the tap function doesn't work. It's very strange. I don't know why. what is the effect in unity when using the collider? if somebody knows,please help me. thanks in advance.