What causes upload error?

Hi. I have a codeigniter website with a strange error.
There's an upload page where the image upload working perfectly but if I put that upload view to another page only the form shows but won't upload. It's not showing error message.
Even if I write code from tutorial the same error occurres. However I know the tutorial is working on my server at a fresh start.
But on my website maybe there's a "hidden" setting which prevents the upload.
Anyone knows a solution?
It's driving me crazy :/