Oh, I know this has nothing to do with your app. I am having the same issue when I go to their National Viewer app: https://apps.nationalmap.gov/viewer/I almost never look at those myself, so no idea what they are usually like. This loads pretty fast initially, but then not all the tiles are updated so I'm getting kind of a "Swiss cheese" effect. Same with the others in the series.
Like I said, their servers can be pretty unreliable. Sorry, it is what it is, this has nothing to do with my app. As I explained elsewhere, when you come to my site, my app is loaded directly into your computer or phone. Then when you choose a map, the data is sent directly to you. My server is not even part of that equation... it's all between your computer and the USGS server.
In a few weeks it will expand to an even larger area that includes all of New Jersey, Delaware, Maryland, Pennsylvania, Connecticut and Rhode Island along with significant portions of other states.