One of our house’s toilets has, after almost eight years of trouble-free operation, not been working well lately. It clogs much more easily, and the bowl doesn’t completely refill with water when it’s flushed. It’s a Toto 1.6 Gpf/6.0 Lpf. I’m no plumber, although I’ll probably be calling one soon. Any suggestions?
Does the tank completely empty and then refill when you flush? Is the float in the correct position? If it’s too low for whatever reason then the tank won’t have enough water to flush properly (when the float is raised up by water refilling the tank it trips a valve and stops water from entering the tank - if the float hits that point too soon the tank won’t be full enough). See if the float has got water in it (I think the fix for that is just replace the leaky float) or if the arm holding it needs to be adjusted up a bit (there may be some kind of screw to adjust it, some other models you just bend the arm a little).
I assume that an ill-fitting flapper wouldn’t cause this problem (or you’d notice the toilet running) but you can replace the rubber flapper for about $5, that might give you a better initial flush.
It’s sick of your shit 
It’s probably the wax plug. One of my toilets does the same thing. My house inspector said the wax plug was old and brittle and should be replaced, otherwise it would clog about 10% of the time. It does. I know I need to replace it, I just don’t wanna deal with the crap (literally).
Do you have small children in the house? It could be a toy or hairbrush wedged up into the bend of the toilet.
i agree that the float valve shutting too soon might be the problem. there may be a line in the tank indicating the level that it should fill to. this would affect the bowl not refilling and not enough water for a good flush. this is the simplest cause.
though other causes like the siphon action of the bowl not stopping quickly would prevent total refill because of a partially blocked waste passage might also cause both problems with a correctly working float valve, though in this case you would see the bowl level get much higher before it empties and it would do so slower without vigorous action and sound.