There are many reasons a toilet will not flush, but this one is unique.
Check out the 3 images below, what you are looking at is the inside of a toilet tank, the item in the tank is the toilet flush valve, when you press the flush handle, the flapper comes up and the water from the tank goes through the flush valve, into the toilet causing the toilet to flush.
Note: I have removed the flapper so that I could get a good picture of
In the first picture there is something in the flush valve and there are air bubbles coming up from the toilet. What the hell is that.
I have to reach in and grab whatever it is, it's blue so I know it is not anything I do not want to touch. Oh it's one of those damn blue water drop-in tablets again!
As you can see, this time I was able to just pull it out of the flush valve with out taking the toilet apart, if it was just a little smaller, this would have turned into a bigger job, costing a few hundred dollars and possibly a new toilet, as it is it cost this home owner an unnecessary service charge.
Ask yourself, Do I really need blue water?
Note: the white bleach toilet tank drop-in can do the same thing, and both are bad for your toilet.