Problem with Parkway

[sueprinter]

I bought the Britax Parkway, I think from Baby Universe. It has the headrest with the sidwings. The problem is with the headrest. When I adjust it for my DS height, it does not lock into place. He can be sitting there, and it just slips all the way down. This doesnt seem right. Am I supposed to be doing something to lock it into place, or is there something wrong with it? I ordered it online, but I do not think I have the receipt anymore. I have had the seat about two months. Is is to late to return it if needed?

Please let me know if I am doing something wrong.

Thanks so much!!