That's simple... one solution is to unclog it yourself. I did that once after some woman who didn't know how to use an airplane bathroom stuffed the bowl with paper towels just before push-back. It was a delicate but I didn't suffer too much. Another time I flushed several times before clearing my problem. And the third way that I will suggest is to simply seek out a flight attendant and say "someone clogged the aft toilet". No need to take any blame, and no need to lie. Someone did...

On the other hand, I'll bet that half or more of the flight attendants you report such a problem to will not deal directly with it. Occasionally you will see a flight attendant deal with a problem of this nature, but IME, not more than half. And I'm not really sure it is their job in the first place.

And lastly, I have been on a couple foreign airlines where I have noted that cleaning the toilet appears to be NO ONE'S job for days and days. Ugh.