Organ effect problems
One of the most common problems of inexperienced orchestrators is the “organ-effect”, which happens when you double too many instruments in a middle register with relatively long notes. At a certain point, the transparency gets lost and the whole texture sounds more like a organ than an orchestra. To avoid that, don’t overdo the doubling in general, some colours sound better unmixed. Also, use the higher and lower registers evenly, try to balance out chords over the whole range of the orchestra and make sure to not concentrate all forces in the middle register. Avoid writing extensive legato passages in all voices. Especially on a big tutti, everybody playing legato will quite quickly create this organ effect. Also, leaving certain instruments out for the sake of bringing them in on peaks is generally a better idea. For example, the organ effect gets highly increased by constant use of trumpets in legato lines. Transparency is key here and when you are not sure why you’re doubling certain things, rather avoid doing that instead of having the fear of “the score sheet looking so empty”.