Most US guitars use an imperial sized key but Highway 1s are mostly made from "international" parts and need a metric tool, either 4mm or 5mm. It sounds like the allen key is slipping in the socket. I'd take it to a pro rather than risk rounding out the socket. It gets expensive after that. If you're anywhere near me I'd be happy to look it over for you. PM me for contact details if you want more advice.