I have a toilet that about half of the time doesn't flush fully, but will always flush fine on the second attempt. It does this regardless of the "contents" in the bowl.
I first took a plunger to it, but it didn't help at all. I then came here and read How can I fix a toilet that flushes incompletely? and looked inside the tank while flushing and the flapper seems to be functioning normally. I then read Why do my toilets flush slowly and often incompletely? and cleaned up everything like Inkspeak mentioned in his answer, but still hit or miss. I am starting to wonder if my toilet is as bad as Inkspeak says one of his three toilets were.
I bought this house just a few months ago and it has been happening the whole time. The house was built in 2007 and this is the only toilet that is having the issue. Is there anything else that I could try before I just replace the whole toilet? It seems like the toilet is young enough that it shouldn't have too much build up in it, but I also know that we have rather hard water based off of how fast we see mineral build up in our shower that we didn't see in our previous house.