When I issue a check payment via QuickBooks (and my bank) who sends the check? I sent a check that was not received and the bank apparently sees this as an ACH payment.

Allow me to share some information about how payment was processed and guide you on what you'll have to do, laurawilker-gmai.

If you're using Bill Pay, this lets you send checks and bank deposits quickly and easily. You'll want to make sure that you've entered your bank account information to pay bills online inside your accounting software so you can settle transactions in real-time. Please check this article for more information: Bill pay explained: How It Works, Benefits, and Setup. 

Also, if you e made the payment by paper check, it will arrive by mail in approximately 5 to 7 business days. You'll have the option to expedite your check to avoid late fees. For bank transfer or ACH transfer, it takes 1 to 3 business days making it the fastest business-to-business payment method offered.

I've also included this article in case you'll want to match transactions with what you've entered in QuickBooks: Categorize And Match Online Bank Transactions In QuickBooks Online.

Keep in touch if you have any concerns with your check payment. As always, we're here to help you.