I've written to Subway in the past. They can take a long time to respond, if at all. Here's why:

Each Subway location is independently owned and operated. So, when Subway gets a letter, they forward it to the location.

At that point, they apparently don't have a mechanism (don't want) to determine if an individual location has responded or not. Unlike the BBB, the act of responding doesn't gain you points (or a non-response loses you points).

An individual location should care, of course, about responding. But they don't always. I've experience (and read enough letters here at PFB) to know that some (but certainly not all) Subway owners don't have the same customer service attitude that you'd hope. I once had a Subway manager say "this is my restaurant, you don't like it, you get out."

If you write to corporate about a corporate item, you also run the risk of not getting a response. Or getting one that's wildly off-base. I once wrote to Subway (anyone detecting a pattern here) about a chain-wide issue. I got a quick response, but it had nothing to do with the issue. When I wrote back, I got a response that apologized, but explained (and I'm not making this up) that she (the person that read my original message) was deaf and this sometimes kept her from being able to respond fully. I'm not sure what deaf would have to do with being able to respond to a written e-mail, but there you have it. She said that because she was unable to handle my issue, she'd pass it along to her supervisor. Who sent me a "Thank you for writing" and nothing more.