Display ad in email

I'm using ad module to display ads in emails sent by Simplenews module. I call ad(12) function from simplenews.module.
When I send test email using simplenews, the ad is displaying all times, but when I send email to a newsletter members, ads show only rarely and most of the times ads are not being displayed in email.
I haven't found out yet why? I think it's not permission problem, because if it was, ads won't display all time, but it shows sometimes.
The message in log is: <!-- No active ads were found in t12 -->
When ads are displayed in email, the email source contains link of ad: http://example.com/sites/all/modules/ad/serve.php?o=image&a=12352&c=file...
With debug=2 I'm not getting anything displayed, but with debug=2 below information are displayed:
ad_dir: '/var/www/html/sites/all/modules/ad'
debug: '1'
adcache: 'fileArray'
nids: ''
tids: ''
hostid: ''
url: ''
alias: ''
quantity: '1'
aid: '12352'
ad_display: 'javascript'

Please help me how can I display ads in email all time?