There's a number of reasons why you are getting error with some posters
1. Site is offline, or was too busy atm.
2. WP updates their structure, and posters haven't got updates yet
3. Badly optimized site, being overloaded, once poster is trying to post on it.
4. Structure/script of site manually customized to prevent bots, which influence on success rate
5. Captcha that is not supported by poster
6. Anti-bot scripts
Basically, there's no poster that can be 100% optimized to work on all possible combinations of blogs, forums, etc...
There is no perfect poster, only the one that fits your needs.