Changes

By going deep [in meditation] one person may see visions; another may fall in deeper consciousness but see no vision—and so on. The result varies with the nature. <ref>http://incarnateword.in/cwsa/30/the-value-of-visions#p8</ref>

Concentration is very helpful and necessary—the more one concentrates (of course in the limits of the body's capacity without straining it), the more the force of the Yoga grows. But you must be prepared for the meditation being sometimes not successful and not get upset by it—for that variability of the meditations happens to everybody. There are different causes for it. But it is mostly something physical that interferes, either the need of the body to take time to assimilate what has come or been done or sometimes inertia or dullness due to causes such as those you mention or others. The best thing is to remain quiet and not get nervous or dejected—till the force acts again.<ref>http://incarnateword.in/cwsa/29/concentration-and-meditation#p109</ref>