If the WD 40 doesn't work I would suggest BREAK FREE CLP which is used for freeing up jammed semi-automatic and automatic firearms. I have used it on several occasions on rusted and frozen screws etc. on my trains and it worked great. For best results apply the lubricant in either spray or liquid form and allow it to cure overnight. Then work the hardware until it frees up. I purchased my supply at Dick's Sporting goods. If you can't get it there try Amazon or any hunting/fishing outlet.
I have used it on firearms as well and swear by it. There is no cautionary statement on the containers regarding being unsafe for plastic.