Leather couch sinks down in the middle

[Prima]

I'm attempting to repair a leather sofa that sinks in the middle if you sit in that spot. The cushions are attached and still very full and plump, which lead me to believe the problem must be underneath? (broken springs or such) I have removed the dust cover underneath and I am confused to find ALL springs are attached and tight(none broken), ALL the wire cables (2)that go the length of the sofa are attached and tight(none broken) and No cracks or damage to the wood!
It's much too large to fit in my car to take to an upholstery place,. which I why I'm attempting this DIY. Anyone have some knowledge about the support system of sofas? This sagging middle spot is driving me nuts! Also, there are these brown(ribbon type) straps that I have no idea what they're use is? You can see in the pics, they are stapled to the wood frame in the exact spot where the sinking problem is. It appears to me that they pull the center seat down. Any thoughts about their purpose? or if I remove them might that fix the sinking

problem?

 

[DaveHerns]

I would guess those straps are to pull the seat cover down so it stays in contact with the foam and doesn't go baggy "upwards" . I have the same problem with my settees but I am overweight .....

 

[Prima]

Dave, that seems like a reasonable possibility. Also, I'm 5'8" 144lbs, so I'm not the reason for the sinking seat, however I have had some guests that are on the plump side.

 

[foxhole]

Strapping prevents the springs spreading , springs may be worn , only snap if really worn. You would need to replace .

 

[freddiemercurystwin]

It's probably just knackered. Sofa's are consumer items.

 

[Prima]

Foxhole I respectfully disagree with your opinion

Strapping prevents the springs spreading

Because there are springs all the way from the left to the right side of the couch. The ribbon straps are only located in one spot, exactly in the center, positioned directly under the space between the to seat cushions above. If their function was to keep springs from spreading, there would be a lot more of these straps I'm sure. Does anybody do upholstery??