I agree Kristen, a paed visit definitely wouldn't hurt, but I think she sounds perfectly normal (probably not what you want to hear). Blake is behind in speech etc but physically is advanced/on cue, considering he was 4 weeks early. Babies develop at different rates and I know you're aware of this, so beating yourself up about it is definitely not going to help at all.
Mia sounds like such a wonderful little girl, if she has no desire to roll or crawl that might just be her way of doing things. As Danni said, Mason is older and is just sitting now, I've seen him in action and he's quite content to lay back and play with his toys, squeal with delight etc. Blake has only just started to be vocal, but doesn't make any sounds like dada or anything, which I know Mia does. I was worried, but I figure he's learning other things phsyically to make up for it. Alot of babies that take a bit longer to crawl usually walk first, maybe she's just building up to that instead of crawling?
Sorry to ramble but I feel what you're going through - all our bubs are totally different, not one is on the same level and I think that's the great thing about them - they are all diverse and we're all having milestones at different times. Imagine how boring the thread would be if they all did things on the same day - no one could get excited for anyone else lol
Chin up sweetie, she will do things when she's ready, I promise