DD2 is 14mo and I just got her down to 4 bottles, so not unheard of!
Can you increase the oz she takes for her other bottles?
The thing that actually sorted dd2 and got her down to 4 bottles was switching to solids first. I hadnt done it yet as she was teething and sick for a couple months and the timing was just wrong, but once I switched she went to 4 bottles pretty easily.