Common problem, especially with this ethanol blend fuel. Your float is stuck on the bottom. You need to clean the shaft that the fuel float rides on on the sending unit. Every mower I've ever worked on that had gauge issues had a gummed up sending unit float. Typically it's just a matter of prying the sending unit out of the tank that's not reading properly and using brake cleaner and rags on the shaft and working the float around till it moves freely. The hardest part is getting the sending unit out because you always feel like your right on the edge of braking it but they are pretty stout and I've never had one brake on me. Easy fix.