post file uploads in apache webserver w/ PHP, sometimes it works, sometimes it doesn't

I have two servers configured with Apache webserver and PHP 5.1

The thing is that I'm trying to upload files to them via POST, in server A I can upload files with no problems, but in server B I can't when files are bigger than 10Mb...

In both servers PHP is correctly configured, php.ini settings for maximum file size in uploads and posts are set to 200Mb, so there shouldn't be any problem, even though, I've noticed that using Firefox I can upload any files with no prob, but using other browsers, IE or Chrome, the problem happens quite frequently.

Is there an Apache configuration or something I'm missing about maximum file size uploads via post? I was looking at error logs but couldn't find any clue that relates to the problem, or there should be?

Also, server B is working behind a load balancer. Can't say what configuration it has, I have no access to it, but could it be a source for the problem?