Can't upload some files for no reason

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PHP

I use a form <form enctype="multipart/form-data"...

to upload files. And then the php code:

if(move_uploaded_file($_FILES['uploaded']['tmp_name'], $target_path))
            {
                echo "ok";
            }
            else
            {
             echo "not ok<br>";
            }

This form so far have uploaded more than 10000 files (.apk type - android apps). Problem is, I noticed now that for some files don't work. I tried rename it for 1.apk (for example), it's not a size thing nor permission (i did a chmod 777 on those test files and on the target path just to make sure), but for some reason, for those files that are absolutely similars to the other ones, but I get the no ok notification.

Anyone is seeing something I'm not seeing ? My problem is really not finding a problem pattern so I can solve it!

I have

error_reporting(E_ALL);
            ini_set("display_errors", 1);

But I get nothing, but still dont know what is the problem. Also tried

if(copy($HTTP_POST_FILES['uploaded']['tmp_name'], $target_path))

but still dont work.