Game hero sometimes can't move on connected blocks

Hello Unity Masters,I connected 4 blocks to a big block,then,the hero can run on it.

But sometimes,the hero will stop when she is walking through the connected place,like the under picture. The connetion is smooth,every block has the same height. Why the hero stop??Sometimes can't stop,sometimes stop.

Hero moving script:body2D.velocity=new Vector2(Input.GetAxis("Horizontal") * runSpeed,body2D.velocity.y); The bolck and hero both have box collider2D Component.

Waiting for reply,thanks~