You shouldn't stop working on a difficult passage after it worked only once.If you played a passage 20 times and only got it right once, that's not a very good success rate! You should keep practicing it until you can play it with ease or at least until it works most of the time.
Sometimes it may take several days or even weeks for a passage to work well so time. patience and perseverance are your friends here.
When you practice a new piece or etude, don't just run through it over and over again. You should also take time to work on each difficult passage thoroughly even if you have to spend an hour on one bar. If you don't do that, you'll still miss the same notes for months.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.