Subtext, especially good subtext, can be tricky to write. But in order to write good subtext, you need to have context first. And in order to do that, you need to understand the difference between them and where each one fits in storytelling.
Some writers make the mistake of trying to make the context into subtext. This is a problem for several reasons, one of the main being that it makes the story very vague. In vague writing, the audience can't really tell what is going on. Without proper context, they aren't sure how to interpret information and actions. Often, this sort of writing manifests when the writer is trying to follow the "show, don't tell" rule too religiously, which usually leads to writing that is too cinematic.
However, creating context does not necessarily mean you have to "tell" straight-out all the time. It can also come from taking advantage of connotations, words with specific feelings attached to them. With that said, though, it's impossible for most stories to have proper context without some telling.