Pretty much the same thing happened here...Cameron crawled at exactly 7 months, but still couldn't sit up. I was able to plop him into the sitting position, but would only last a few seconds or a little longer if I put him in the couch I bought him for xmas. Then only 3 weeks ago (just over 8months) he went from crawling position to the sitting position...I was so excited I started clapping and cheering - it just happened out of nowhere. Don't worry about it too much, it'll just happen one day when you least expect it.