Vibe buggy

Our not so wee fella is almost 6 months old and weighs 9.3kg (solely breastfed) and when I adjust the back of the buggy so he can sit semi upright the backboard bends and pushes his head foward.. we had the whole thing replaced last month because of this and have just noticed it's doing it again. I think the problem is that he's very heavy but is still small so the weight is not being supported by the strap on the back because it's inline with his head still and not his back. I've called them again and am now waiting for a callback but I'm thinking I want to get a different buggy as this will probably keep happening.
Has anyone else had this problem with the vibe.
We got the vibe as we wanted an inline double and this was pretty much it. Are there any others?