I have had the same issue (my card apps showed successful payment but no Avios were deducted and no booking made by Qatar and then the payment rejected by Qatar).
I tried to pay with Amex first. After that with a Mastercard. same result. The problem seems to be that due to the extra security checks my card issuers use, the payment is rejected by Qatar. An agent said that the payment and the Avios need to be processed exactly at the same time or otherwise the booking will fail.

Every time my booking failed, my seats were gone anfter my failed anttempt and only double Avios were available. After calling and speaking to an agent they were able to find my booking attempt and were able to release the seats again (did this 3 or 4 times in a row).

I finally succeeded with using the app and paying with PayPal (no security checks there).

all in all a very frustrating process that took me hours but I finally managed to book the seats. Hope this info helps some others.