When Some Lyrics are Better than Others

Let me preface this by saying that I am not new to songwriting. I have over 7 years of experience and I'm (mostly) proud of my writing, but the biggest hurdle I face is filling in the gaps. I have about 30 ideas which seem extremely promising, but I've only finished 2 songs. My best work comes to me when I don't force it, but waiting for the perfect lyrics to come won't cut it when I'll eventually have deadlines. When I do sit myself down and attempt to finish, I find that even within the same verse there will, at times, be an equal mix of brilliance and utter crap hahaha. Seriously. I don't understand how some of the best lines can come to you instantly, yet when there's a gap, you sit for ages and what you do manage to pull together doesn't measure up to the rest of it. How do you resolve this without settling for mediocre lyrics?