There are a few common reasons why custom php.ini files won't work.
If you are still having difficulties, please contact support. Make sure you include any settings that you are attempting to modify, the expected results of the setting along with the actual results.
Often if you're saving it in Windows - for example with Notepad - the file may have been accidentally named "php.ini.txt". Please ensure this isn't the case.
php.ini files in child directories take precedence over any parent directories. Make sure you don't have an additional php.ini file which is taking precedence. Best practise is to include a single php.ini file in your public_html directory, this will affect all PHP scripts within your webspace, including all subdirectories.
This is very common within certain CMS systems. If you set one memory limit within a php.ini file, and another within your site's code, the latter will take precedence.
If commands are misspelt or malformed, they will not function correctly. Consult http://www.php.net to ensure that your commands are correct.