"The more you look around at things, the more you see. The more you photograph, the more you realize what can be photographed and what can't be photographed. You just have to keep doing it." ~ Eliot Porter
GOOD ADVICE. SOUNDS SIMPLE ENOUGH BUT.....the implied idea that some things "can't be photographed" is, iMo, a two-sided coin inasmuch as there are-at the very least-2 aspects of a picture to consider. That is, the tangible (aka: the depicted content/referent) and the intanglible (aka: the intended concept imparted by the picture's maker).
Consequently, I believe that just about any thing or every thing a camera can be pointed toward can photographed-specialized referents may require specialized gear-however, try as a picture maker might, it is not always possible to "capture" an intended concept. Or, in other words, it is almost always possible to illustrate a referent but not so easy to illuminate a concept (an expression of a picture maker's vision) associated with the photographing of it.
In any event, the suggestion to keep on trying is damn good advice.