Problematic phone being rejected for warranty repl

I bought a Samsung A5 thru contract a few weeks but in about a hour started experiencing an intermittent issue with the back button where it doesnt work/respond. I sent it for replacement under warranty but then it was rejected because they couldn't find the fault. So I picked up the phone yesterday afternoon from the shop & used it in the evening but then early this morning the issue started again. WHAT YOU NEED TO TAKE INTO ACCOUNT IS THAT THE ISSUE SHOWS UP INTERMITTENTLY/FROM TIME TO TIME AND IT CAN TAKE SEVERAL HOURS OF USE FOR IT TO COME UP & AFTER A WHILE IT STARTS WORKING AGAIN, & this was stated on the documents. I don't know how long your assessors test the phone & I'm guessing it's not for long enough for the issue to show up. When I sent it the store member who booked it in even witnessed the issue & if you want you can the store and speak to him to confirm. I wouldn't be sending back a phone inconveniencing myself with having to go back & forth to the store for no reason. I'm taking it back again for 2nd time to the shop & I hope this doesn't become an endless case of ping-pong back and forth in sending the device back which is surely a major waste of my time & yours.