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No. The casing does not matter.
First thing to check would be the spelling of the username. Make sure you did not prepend the u/- suffix on usernames. If it still doesn't work, you may have been shadowbanned from Reddit.
We are currently working on the database. As a result, queries are not cached yet (i.e. processing will occur again even if you search the same username twice).
To prevent server timeouts and overload, the retrieving of submissions and comments have been made client-side. It is your machine that connects to the Reddit API. Thus, the speed of data gathering is dependent on your network speed. In addition, the Reddit API rules state that no more than 30 requests per minute should be made, thus RedditMetis cannot retrieve all the data at once.
Metis, in ancient Greek religion, was a mythical Titaness which, by the 5th century BC, was regarded as the mother of wisdom and deep thought. You can probably see how this matches RedditMetis' data scraping capability. Coincidentally, Metis is the name of an asteroid first observed in 1848. An asteroid was initially considered to become the logo of RedditMetis.