I can sympathize with author Miki Yoshikawa's dilemma. She was a lowly 23 year-old assistant on "Fairy Tail" when she got a chance to write a one-shot in 2005. Yoshikawa knocked it out of the park, coming up with a creative, funny scenario involving two oddball, likable characters in Shinigawa, the "Yankee-kun" of the title, and Adachi, the "Megane-chan". The bespectacled heroine tries to convince the school delinquent, camping out in the bathroom, to go on a class field trip.
After this initial success, Yoshikawa was offered her own series based on the one-shot in a popular magazine at only 24. And what do you know; the duo went on more funny adventures, and new, amusing characters were introduced to the cast. The manga was well-received and its sales were excellent.
Eventually, however, she had run out of ideas for humorous adventures. And yet, every week, there was that brutal deadline for 20 fresh new pages. What to do? Canceling a popular series, especially the first hit for a young writer, was unthinkable. So she soldiered on.
Storylines were repeated. New scenarios were nonsensical pap that led nowhere. In an effort to break up the monotony, many new characters, including underclassmen, were introduced as the main cast aged.
They all invariably sucked, with each being an increasingly paler imitation of the main five introduced early in the series. But they weren't the only lousy ones. Instead of developing more personality and complexity, the central characters all became increasingly trite and one-note. For instance, Izumi by volume 14 was a bland, irrelevant character despite having been an amusing, colorful scene stealer when first introduced.
I noticed this very clearly. A manga that I would look forward to and laugh at gradually became dull when its plot holes and non-existent motivations weren't making it annoying. And as it continued to degrade further, it became outright torture to complete a chapter.
While many readers noticed this huge drop in quality beginning with volume 8 or so, many others continued to enjoy it.
However, it all come to a head with the manga's conclusion, one so dire, lazy, and insultingly stupid that even the most ardent defenders of the series cried foul. Personally, I could almost feel the author's hatred of the characters and desire to be rid of them. Having them all die in a tragic mercury poisoning accident after eating at the same sushi restaurant would have been more merciful than their actual fates.
If nothing else, the last 16 volumes of this manga indicate the flip side of any series attaining success. Namely, the unrelenting pressure to keep going. Yoshikawa should have never let it get to that point, but as a first-time writer, one can empathize with her plight.
My advice? Read the first 7 volumes, enjoy them, and pretend that's where the series ended.