Copyscape premium fails to deliver- Concern and question

[Sajiali]

Recently i have been using Copyscape premium to check the outsourced worked, but on many times it didn't caught the duplicate/plagiarism contents.

One article run through copyscape get passed without any plagiarism and when i recheck the same article second time it caught plagiarism with five different sources. I am really concern about this, does some one else find this thing with copyscape?

Copyscape have set the content limit to 2000, but what is the ideal length to run through it?

I am also using palgium and grammarly for the same purpose. What other reliable alternatives are available to use?

[CoreyNorthcutt]

I often feel like Copyscape was created by the owners of automated content spinners to falsely prove themselves legitimate.  The place that I've seen it talked about, by far the most, is in any spammy blackhatworld.com or warriorforum.com offerings, talking about how they pass Copyscape.

You should find that quickly, Google is a much more accurate at understanding similar copy, if you paste a few lines into search and thumb through a few pages.  I would never trust Copyscape alone.  There are a number of reasons that it may be wrong; as there's definitely a lot to consider when analyzing copy.