Apparently, Yahoo has made some changes to Reminders. For example, they no longer provide text messaging to mobile devices. Only thing I can suggest is to disable your current settings, log out of Yahoo if you're always logged in, and then log back in and re-enable your Reminder settings.
Computer type Laptop System Manufacturer/Model Number Sony Vaio VPCEB47GM Laptop OS Win 7 Pro 64-bit CPU Intel i5 2.4 Ghz Memory 8GB DDR3 Graphics Card Intel HD 3000 Sound Card IDT High Definition Monitor(s) Displays 15.6 WGXA Anti-Glare LED Screen Resolution 1280x800
Hard Drives 640Gb 7200rpm Antivirus MSE Browser Opera (primary) with IE9 backup