Because an organ usually has far more flue pipes than Vox Humana pipes, for practical purposes we simply retune the Vox Humana even though it probably hasn't gone out of tune. Reed pipes require more attention, simply because they are more complicated mechanically, and thus are more subject to derangement, and also because of the different way they react to temperature changes. However, simply tuning the reeds by their tuning wires can derange the regulation. The regulation depends, among other things, on the relationship between the tuning length of the resonator and the position of the tuning wire on the tongue. All other things equal, if the tongue is lengthened without shortening the resonator the tone will get louder and brighter, and vice-versa.
Reeds are also quite sensitive to dirt or insects getting under the tongue.
This often silences the pipe and requires that it be expertly cleaned to restore its speech. A well-designed heating system can guarantee this temperature in winter, and most American churches have such a system. Many, however, particularly in cooler parts of the country, do not have air conditioning and thus temperatures may go higher than this in the summer. For these environments it is fairly easy to insure the tuning of the organ through the winter, but the reeds may become a problem in the hot summer months. If the church has a slack season in the summer it may be the best course to simply avoid use of the reeds on hot days, knowing that they will be in good shape when cool weather returns.