I have two screen grabs attached to show what I'm talking about with this.
I started a drawing for a project, and so far I don't have much information in the drawing. I created a few objects, then went to one of my Layout pages and created a viewport to view the objects, and bam - some of the lines are missing (the yellow objects). As you can see on the screen grabs, I hovered over the Model tab to prove they're still there. Even when I double-click inside the viewport to edit through it and try to grab the objects, they're apparently gone. But they're not, they show up in the Model tab.
The layer they're on is turned on, thawed, and unlocked, but every object in that layer is missing when I leave the Model tab. I've opened viewports on other Layout pages, and they all refuse to show the yellow lines.
Are the layers thawed inside the viewport? It's a separate setting from thawed/on for modelspace and you can only edit these columns in LAYER palette/command window when you are inside the viewport. See attached.