1). The format of your document is not compatible with iPlagiarism. The simplest solution is to convert your document into txt (Plain Text) if the format is the reason.
2). The file name is too long. The simplest solution is to rename the file before detection.
3). The document must be less than 100,000 characters. If your document is larger, you can split it in parts of 100,000 characters or less.
4). Another common reason is that a user uploads a document with the same file name as a previous document. To solve that, you just need to rename the second document and upload it again.