However, as the quote goes, "That's the problem with first impressions. You only get to make one." People tend to be on their guard when interacting with someone for the first time - even if that interaction is just reading a cover letter. Things that your friends wouldn't blink an eye at can easily become reasons the hiring manager tosses out your resume, and unfortunately you rarely get a chance to explain yourself or even feedback that you did something wrong.
The best part is that fixing this doesn't involve a certification program, or spending more money, or anything like that. Maybe just have a couple people read over your cover letters or do mock interviews with you (not sure where you're getting stopped in the process, so can't give specifics there). Maybe try it with someone not super close to you (maybe a former co-worker?) who'll bring up something you're saying or doing that might seem innocuous but is raising alarms somewhere.