You've edited for content, checked your characterization, made sure the timelines work, and read your dialogue to the cat, who nodded agreement. There's one more thing that can shoot a good manscript out of the sky, and that's copyediting. We've all seen instances where that one wasn't done quite well enough, but one must try.
I've heard the arguments. "I can't find my own mistakes." "They pay people to do that." "I'm not a grammar expert/great speller/word Nazi." While these may be true, there's the other side of it. First, you can find a lot of your mistakes if you try. Leaving a work alone for a long period of time (time in a drawer, to misquote Jim Croce) gives you a fresh eye for errors. Reading aloud forces you to look at each word. That lovely little underline feature in your computer tells you what it thinks might be wrong. It isn't always right, but it alerts you to possible problems.
Secondly, while there are paid editors at publishing houses, they have plenty of people who send them nearly perfect manscripts. Therefore, why should they bother with yours if it has three typos on the first page? Every mistake you leave in raises your risk of rejection. That editor has dozens of other scripts to look at: we don't want to irritate her.
Okay, I'm an English major with thirty years' experience in the field, so I'll admit that I have an edge with finding mistakes in copy. You may not feel confident with the whole pronoun thing or clauses that refuse to be subordinate. In order to let the agents/editors know that you are a serious writer, you may have to pay someone to edit your work. It's well worth it if you find someone who is good at the job. Volunteers are great, but make sure the person you choose is committed and competent. My sister, for example, would love to help, but she has two problems with editing. First, she thinks like I do, so when I make a mistake she intuitively knows what I meant and reads right over it. Second, she tends to get involved in the story and forget that she's looking for mistakes. Neither of those is a bad thing in my opinion, but it doesn't get the job done, so I'd look elsewhere if I needed editing.
We've all seen funny examples of poor editing. One of my favorites was caught before we published, but in a flier several Five Star authors were going to send to libraries we found the phrase, "so he was reluctant to go into pubic areas." It took four professional authors' editing to catch that one.
So edit for mistakes. Then edit again. Another time wouldn't hurt. Another set of eyes wouldn't, either. The last time I read through Macbeth's Niece, on the ARC level, I found a dozen errors that I'd missed at least ten times, and my agent, two editors, and a copy editor had missed each in turn. Luckily, the organist at my church found them. Now who would have thought to ask her? But don't imagine I won't think of her next time.