Most RSS feeds have only the most recent 10 articles in them by default. If all those articles are in the mailing, FeedBlitz will send them. But if there are older articles that you wanted mailed but did not go out, it is probably because they “fell off” the end of the RSS feed by the time FeedBlitz checked it.
The solution is simple if you can’t or don’t want to change the delivery schedule to something faster (at Newsletters | Mailings | Schedule) – make your RSS feed longer.
In particular, if you have a weekly mailing, ensure that the feed is long enough to contain the entire week’s posts. Similarly for monthly mailings – the feed should be long enough to contain a month’s worth of posts. This is because for a weekly mailing, FeedBlitz checks your blog once a week, just before it sends out the mail. Similarly for monthly schedules – your content is checked just once a month. Even for daily updates, if you post a lot during the day it is possible that posts for the daily email update have already been dropped by the feed by the time FeedBlitz checks for them for the daily delivery.
The setting for RSS article count varies by blogging platform – if you cannot find it easily a quick search online should find the instructions. For Blogger and TypePad the setting to change is the number of articles to show on the home page. On wordpress the feed count setting is in the “Reading” area of the main admin page.