Are you tracking any other metrics besides response rate in order to determine if they are engaging with the emails that you have sent? As others have recommended, the best idea would be to remove the email part from your strategy, at least until you get a better idea of why your emails aren't working.

Also depending on how long ago they subscribed to your list, you may just be better offer culling them from the list. After a certain time, it can incur more costs than it is worth to try to reactivate inactive email subscribers.

I'd be happy to discuss some options with you after I get a better idea of your sales funnel goals.