Need help with down sofa cushions that don't hold their shape.

Hi,
I have a sofa that I designed & had custom made with down filled cushions. I have an issue with the back cushions holding their shape. They have a tendency to slouch down & get fat at the bottom. The covers are removable & the inner "bag" is also zippered. I am looking at 2 possible solutions, but want a sanity check since I have no experience in this area. I can add tufting to pull in the front of the cushions & hopefully they will retain their height & proper thickness or I can add firm foam up the center of the filling to add some structure. I guess I could do both? Any insight will be appreciated. The cushions are about 38" long x 18" high x 9" thick.
Thanks,
Kev

Pull them out of the casing, fluff them up by hand and re-install. Repeat as necessary - that's normal for Down Filled Cushions.

If you tire of that, have the sofa maker build you some Fiber fill Cushions and insert those instead.

That happened over time on my last sofa. Do you recommend doing that with the back cushions on the City Sofa as well? Or just ask for new cushions if that starts to happen?

All down cores are going to do that. If you don't want to have to fluff, don't check off that option box.

Thanks for all the replies. My back cushions are made with 3 chambers. There is a front pillow & a back one with a separate "pillow" in the center. all in the same bag. Sorry, don't know the lingo. That allows me to experiment a bit with foam in the middle section in place of the down filled one.
Thanks,
Kev