This has also happened with me on server’s the people over there told me to be more closer to the block while placing them and it actually worked, I don't know whether it's the same problem with you.
It may be because of those cool water reflections, try the settings for vibrant visuals it may help out.
I think this problem can be fixed in the settings for vibrant visuals.
Try the settings that's provide in game for vibrant visuals to match the standard of your device. This can also happen due to high render distance.
Please provide a detailed description
Are you a moderator?
May depend upon the geography of the biome, slime’s only spawn between layers of 51-69, and between light levels 0-6.