Everyone has a problem?
Or do some appear well, and others do not show up well?

These here:

https://www.themoviedb.org/movie/501555-this-world-won-t-break https://www.themoviedb.org/movie/514412-sam-and-gus

I will assume that your answers to my questions are:

Q: Everyone has a problem?
A: No. Only the backdrops I uploaded.

Q: Or do some appear well, and others do not show up well?
A: No. All the backdrops I uploaded do not appear in letterboxd.

I do not know when you uploaded the images.
I know there is a delay for the images to become available.
Some things take 6 to 12 hours.
But I think only Travis Bell can answer your questions.

Or maybe someone from letterboxd.

It seems that this problem is recurrent.
And it seems that only depends on the admin of the letterboxd.

http://feedback.letterboxd.com/forums/136771-general/suggestions/18410935-make-it-easier-to-load-backdrops-onto-films
http://feedback.letterboxd.com/forums/136771-general/suggestions/3755286-make-it-easier-to-get-backdrops-added-to-individua
http://feedback.letterboxd.com/forums/136771-general?query=backdrop

And it seems that only depends on the admin of the letterboxd.

Yes, the Letterboxd staff have full control over backdrops on Letterboxd. Nothing that can be controlled here, other than adding high quality backdrops and suggest them to LB for a movie.

How do I suggest them?

How do I suggest them?

I suppose here
http://feedback.letterboxd.com/forums/136771-general
In the green field > I suggest you ...

Creating a new topic on the LB forum is surely not the correct way. Try some of the options here instead.