Getting Overhead

I feel like Cams' progress has started to slow down...I don't know if it's because we stopped doing so much more therapy at home or what but I'm starting to notice a difference. She's still not getting overhead completely and it's really starting to bother me. She'll lift both arms and obviously her left doesn't go up all the way and just goes past her shoulder height a little. She has to manipulate her whole entire body so it even looks like she is getting that arm overhead. I don't know if this is something that will never happen or if she just needs to get stronger. It's just upsetting. How much can you push a 18 month old?...Especially, when she's already started acting like a terrible two and is very opinionated.