It's difficult to know without seeing the city so I can only make guesses.

1) Is there something wrong with the circuit, so market sellers don't pass those houses, missing road tile? roadblock?

2) Does the market run out of food before the market sellers reach those houses?
Sometimes in a large block, one food shop is not enough to supply all the houses. Place a second food shop.
I always use a Grand Market in a common housing block so there is room for extra shops and you get an extra cart for distributing food

3) some other essential service not passing those houses, like water carrier?

4) Problem with desireablity, those houses too close to something ugly? (seems unlikely)

5) Muggers robbing the carts (you'd hear the moans) also seems unlikely.

Maybe someone else has an idea.

Serrataur gave a great reply, and my guess is that some essential walker is not passing by the houses on a regular basis, therefore those houses are devolving.