Suddenly became exremely difficult to start. Fuel ok. Battery fully charged. engine turns over like normal on the starter but just wont fire.

Try to perform these :
1. Check the sparkplug if it is good or not,.if it will
ignite when you are going to start.if it doesnt ignite,replace with new one.but when after replacing it is still does not ignite,then its time to check the primary coil.You can check it by looking the wire coming from the said coil through the CDI..(mostly it is a black color with a little stripe of any color.) if you already found the said wire,start the motor and ground the said wire to the body with on & off motion.If while doing this you can notice a spark coming from it,then your primary coil is good.,Thus,its time to check your CDI or need to replace it.
But,if there is a fire in the sparkplug when you start the motorcycle but it doesnt work or fire at all,then you must clean the carburator.
Onthe other hand,if your motorcycle is a dynamo/contact point operated,you must check the "timing" into right settings,or give an importance to the condenser if it has a leak and may cause short during the motor will start..