I've had experiences where it went through one time, then the next day, same register, it didn't. My phone would say sent, but the register never got the payment. I went into a CVS one day and there was nobody else in the store, so I asked the clerk if we could test a few transactions to figure out how it worked.
-If she pressed total and didn't press anything for form of payment: when I put my phone up to the pad, it would say sent, but then come back and say it didn't go through, and keep looping. Once she pressed the credit key, if my phone was still touching the pad, it would go through.
-If she pressed total and credit: Went through on the first tap.
-If she pressed total and debit: Phone said sent and that was it, but she never received anything and money was never deducted from my account.
I think it has to do with the handshake between GW and the merchant account. I theorize that the transaction goes something like this:
-When you tap your phone, it gets a code from the pad -> GW sends this code and the authorization to withdraw from your GW account to the merchant account through the network.
-If the clerk doesn't press anything, GW sends your info -> merchant account replies with, nope, we don't have anything matching that yet... try again -> loop sent, error, sent, error, etc...
-Once the clerk presses credit -> merchant account recognizes the authorization from GW (has a sale to apply your payment toward) and the transaction goes through.
-If the clerk presses credit first, the merchant account is already waiting for the information, hence the transaction is approved almost instantly.
-The merchant account is usually the same for debit and CC transactions. Pressing debit tells the merchant account to wait for payment -> merchant account accepts payment -> register system doesn't recognize payment because it's waiting for debit, but getting credit -> merchant doesn't get confirmation the sale was completed from the register -> no funds are withdrawn.
Basically. Long story short. Before you tap your phone, tell the cashier it's credit. Since I started doing that, it's gone through every time.
As a side note, I have had it work fine without the cashier pressing any keys. I think those registers may be setup to automatically detect the form of payment from the reader and would only need the clerk to press something if it's a cash or split cash/(debit/credit) payment.