[Plugin: Contact Form 7] File upload problems

I need to make it possible for site visitors to upload files up to 15 mb but this is not working. for me all files up to 8 mb are uploading and send (works fine), other testers had problems even uploading 6 mb files …. i have the impression that there is a kind of time out when I watch the activity monitor on my computer while uploading. in the forums i read that the php.ini file should be located in the root folder of the WP site, is this correct?

I tried the following/more information:

– the server accepts uploads up to 20 mb size
– max_execution_time was set up by the provider, no changes
– no error code, only the wheel is spinning
– provider told me to set the limit even up, so I set it to limit:30mb, no changes

my code is:
[file* FileUpload limit:15mb]

wordpress 3.0, plugin: contact form 7 Version 2.2.1
I appreciate any help or suggestions! thanks in advance.
c.