Occasionally this is caused by technical problems, but 9 times out of 10, this is caused by spam filters.
You can prevent this in the future by adding "notifications@sifterapp.com" to your address book.
If that's not the case, our next recommendation is checking with your IT department to make sure that their spam filters aren't preventing the email from ever reaching your inbox to begin with.
Another rare but possible cause, if you're forwarding one inbox to another, is that the notifications are getting caught in the spam folder of the email address that is being forwarded. In this case, it won't show up in the spam folder of the email address to which it is forwarded because it was caught in the first email account's spam folder.
If none of these solutions work, you can always shoot us an email and we can look into it further.