Gail refused to accept that the part is finished.

I bought a bicyce last year 26 September for my son. The bicycle was giving me head achs because i have to fix this, bought tubes. Last week Friday i took the bicycle back because the chair is not longer attached to the bike. It is too risky to allow my son to ride without a chair. I went to customer service and they send my to bicycle department to check. I meet Gail and I explained to her want is wrong. She told me about i warranty for which is stated on the bicycle manual. She said i must wait for someone to check it. I waited 10-15 minutes for this guy. He came and took the stracture of the bike to try to fix the chair. He tried and i said he must adjust the chair he told my it cannt because the thing that tighten the chair is finished. Gail didnt want to hear that, she said it is fine and she tried to adjust it herself. I did understand why i was waiting for this guy if she knows what do. I also didnt want to hear that, knowing the fact that chair keep on falling. if my son want to adjust it higher and sit the chair goes down. We also tried to tighted but the same thing keeps on. I am not happy and satisfy about the service I received. I told Gail that im going to complain