(Years ago when I had an piece played by a sub-professional orchestra, I showed the score first to a famous orchestral composer and asked for advice. This esteemed personage suggested three changes – two of which didn’t work out in rehearsal, and I had to retract them.)
The problem of writing orchestra music is the same as writing any other kind of music: fashioning a continuity in which the ideas make themselves clear, take time to breathe, and lead from one to another with a plausible logic, resulting by the end in a meaningful and satisfying large-scale shape. Errors in these areas get writ large in an orchestral format, but the orchestra also provides lots of colorful toys to compensate and distract with. Places where the orchestration seems awkward are almost always places where the musical idea wasn’t well thought through, and wouldn’t have worked any better in a string trio. Writing orchestra music requires a ton of work in checking parts, deciding whether to go solo or “a 2,” and so on, but the common idea that only those in some upper echelon with special experience should be trusted with an orchestra is ridiculous.