Sometimes you will find it is very strange that a few blocks are missing in the layout viewport - When you go back to the model space, they are perfectly there.

You have tried almost everything, including turning on, thawing all layers, thawing all layers in the current viewport, changing the block to another layer, you have even tried using block editer to change the components of the block to other layers - but all failed.

I assume the reason is related to annotative property of the block, or "display scale" of the block which is not shown in the object properties control panel.

Acturally the block is there, but is scaled in the viewport to such a big size that it is "missing" from current viewport.

The simplest way to get it back to the viewport is redefine the block.

Do not use the block editer.

Insert a new block, draw a temporary line from the insert point, explode the block and use command BLOCK to redefine it to the original insert point.