According to the CDC, crawling isn't even a milestone anymore so don't worry too much. My son army crawled for so long and then one day he just switched out of the blue around 9 months. You could try some games/exercises to strengthen the arm muscles. Try google for some options!
My little one is 8months and still army crawls but what I do to help him learn that he’s slowly catches on with it, I prop him on his knees when he’s trying to crawl and he only uses his knees when on the bed
My lb is now 11 months old and he still army crawls, he can crawl normally because he does it on our hard floors but I think he just prefers to army crawl 🤷♀️ he’s actually much quicker when he’s army crawling so that’s probably why 😂