Validation is not run

[eluser]noonereally[/eluser]
HI,

I encountered strange problem.
I have a form with a lot of fields and some of them are for file uploading(input type="file").
And if try to upload few big files - CI just redisplays the form and shows NO errors.
And validation-> run - retuns FALSE.
I'm trying to upload few images,which individually do not exceed the predefined size in upload.php in config folder.

The question is how do i determine the real problem?
validation_errors() - returns empty array, $this->validation->error_string - returns empty string.

How to debug validation? Or maybe it is upload problem?