Could you perhaps make a resin cast of the suspension and thus have a harder more durable part? I'm just trying to think of things to salvage the situation...
I do know the frustration because I felt the same earlier with my Haunebu II from Squadron, which they refused to sort out. Tamiya is a different story and all I've heard is that they're very helpful!
I hope you pull the boxes out of the bin and give it a think instead of giving up. There must be some way to fix the dodgy part?!
Jim GET THEM OUT THE BIN if all else fails make them hard tails (no suspension on the rear) you're giving up to quick there must be a way, look for other build logs of this kit. It's a tamiya so there won't be a problem with the kit. Maybe it's the instructions which are not clear and you are trying to attach bits wrong , as I said if all else fails hard tail them.