Check the value of LimitRequestBody
in the Apache configuration file, every request larger than this limit will be blocked, PHP will not even reply. After you adjust the value reload Apache, otherwise the change will not apply.
Docs: http://httpd.apache.org/docs/current/mod/core.html#limitrequestbody
By the way: if you check the response headers to the upload request, when hitting the server limits you should receive the 413 - Request entity too large
error, so as previously suggested the error and access logs of Apache can be usefuls sources to track down the error.