I can't pinpoint whether it's the cast, casting director or director to blame for the leads failing to make any real impression, so I'll simply say, as an actual mother, if I was in Mia's position, I would be UGLY screaming, there would be snot. I would be in abject terror, to learn that my baby was posessed or missing.
Annabelle (the irony is not lost on me) Wallis comes across as too demure to have even made a baby the biological way, and when she is supposed to fear for her baby's safety she is not in any way convincing; she comes across as weak and hysterical, rather than desperate but powerful and determined (which is what actual mothers are). Her hair is never out of place, her face always looks toned down from the emotion it should be displaying, and her husband is a wet lettuce.
It would be a much better film if the characters were believable. Like, your only friend just sacrificed herself in a horrific way, and the second you see your baby behind you you forget? Guys, it is possible to experience multiple emotions at once! Not to mention it made that woman (of colour) look basically dispensable. Oh wait... just like in every other horror movie where POC get killed off to fulfil an unremorseful quota!
On a related note, regarding credibility:
1- pregnant bellies don't disappear moments after birth. Your body doesn't look adolescent the second the baby is born; John should have known something weird was going on when he walked into the hospital and found Mia sitting in bed looking like she'd never been pregnant.
2- Being stabbed in your pregnant belly doesn't make you look mildly bewildered (but still beautiful) like a Calvin Klein model; where the hell is the acting at?
3- That's not how the cervix works. That's not how any of this works. Please, Hollywood, start consulting midwives before perpetuating ridiculous (and frankly misogynistic) tropes about pregnancy and birth, it's the 21st century!
I tried to like this movie, because I liked The Conjuring (1&2- actually the British actors helped 2 out a lot!) but it's less than tepid, for me. Don't rely on CGI and forget your cast.