Maybe not enough salt to water ratio for the brine? I had an 18 pounder, too. Used about 1 and 1/2 cups of salt to 1 cup of sugar. In my 4.5 gallon "measuring cup" (yep-it is big & has 1/2 gallon marks) it came up to just over 4 gallons to cover everything.
Weber Genesis, CyberQ Wifi on a BGE XL
Miss Silvia and Rocky