My DS is 11 months and a week or so. He bum shuffles rather than crawls (although can commando crawl) and has good fine motor skills. If you stand him up against something he bears his weight independently and can even cruise the sofa a little bit. But he cannot pull himself up. He tries and just gets a bit of the way then sits down and cries. He's not been a particularly fast developer movement wise but surely he needs this step? Can it be a particular muscle problem?