No need to DV them again. They aren't required to respond within 30 days. They aren't required to respond EVER. What they ARE required to do is stop attempting to collect on the account until they have provided validation.
I have one old credit card account that has been through three different collection agencies. Each time I DV an agency, I never hear from them again, but a couple of months later I get a call from a new agency that now has the account. So I guess a lot of agencies take the DV as a sign that you are going to be "difficult" to collect from, and send the account back to the OC.