Yup, that happens to me too. I think it's knowing the word's meaning but not using it enough in practice so then when you need it, it doesn't come easily to you.
I actually use that word a lot. It's just I don't write it out or read it often anymore so when I do write it and pronounce it as I write it, I'm like, oh...that's how you spell it? And, it's not like I've never spelled it too. It's such a common word so it's weird.
There are even times when I can say a word in my head and it sounds right but when I say it out loud, it does not sound right. And, it's not that I can't say it. It's just I haven't said it for so long so I've forgotten how to pronounce it. If I could find that word, I'll share it. Right now my brain is cotton.
And other times the words you use all the time slips from your mind too when you need them most, for some odd reason. Sometimes I forget people's names that I see every day. Memory loss. Scary!
Yes. Insinuate is one of them...LOL. But, now I will never forget it. I've stamped it in. I have a difficult time with people's names as well. I hate introductions because one second after I hear their name, I forget it. I wonder if it's anything they are putting in our water. hmm.