Sounds like a heater cartridge is dead or dying. They have a finite life and need to be replaced every so often. Search Amazon for V6 heater cartridge. I’d go 12v 40w or higher. Keep a few on hand. This isn’t an uncommon issue (with all 3D printers).
The ceramic filling in the heater cartridge cracks. This allows the coil in the cartridge to corrode and leads to slow heating, inability to maintain temperature and if you persevere long enough, complete heater cartridge failure.