I've had this issue since 2018!  Merch Services and QB Payment support never have an answer.  But I have noticed a few things over the years. When I run  ach on customer account that had in the last week or two had nsf's or fraud, I have to wait a few weeks to run again.  But when customer has funds in the account it is just a weird glitch that never gets fixed. So frustrating. And embarrasing when you have to call customer and say payment declined and no one can explain or fix.  

I'd be weary of canceling the payment account. Even if signing right back up.  Glad it worked.

The check is run through the National Check Database.  Your theory sits with what I was told back in 2018 about this.  Our tenants are frustrated as well.  THey call their bank and the bank says that nothing even atempted to run an ACH check.  My client is fed up.  So now we are giong to start shopping other ACH e-check sources.  

The only way I finally get it to work is to either keep hitting the ACH. Because like you said. Their bank doesn't even see it being attempted to be hit. After about 3 or more fast attempts it finally  goes through.  Some I just have them to either the ACH themselves.  But yes it's a Bword@