NexGen Gallery not importing all pictures

Contact you host and review this with them:

http://www.php.net/manual/en/ini.core.php#ini.max-file-uploads

You may also need to raise anyone of these values in your php.ini file or server env:

max_input_time = xx
max_execution_time = xxx
upload_max_filesize = xxM
post_max_size = xxm

You can check these with a phpinfo.php file. It will contain only this:
<? phpinfo(); ?>
Put it in root of host account and navigate to it in a browser //yoursite.com/phpinfo.php