I enjoyed XII, but I have to agree that XIII and XI weren't as good. And it pains me to say this, because I really like this movie, but I don't think XIV held up to previous sequels either, quality-wise. Don't get me wrong--XIV is still one of my favorite LBT movies! I just can't help noticing that the animation wasn't half as beautiful as what we were getting in VIII through XII. Aside from "Look for the Light," the songs were also bland and forgettable. (Yeah, call me crazy, but I thought XIII at least had some catchy tunes going for it.)
LBT VII might be my personal favorite, but LBT X, in my head, has always been the ultimate LBT movie. It has the most interesting Mysterious Beyond environments the series had offered yet. It has the most groundbreaking and complex story of the whole bunch of sequels. (Littlefoot meets his dad?? That's some serious stuff!) It has the most dangers, the most sharpteeth, the "Adventuring" song that can basically be used as a theme for the whole series, the most beautiful animation...it's all-around epic! It was bound to be a difficult act to follow, and maybe that, in itself, contributed to the later drop we perceived in LBT's quality. (Not saying that means we imagined the drop, just saying that, by being an exceptional LBT sequel, X might have exaggerated it.)
Plus, as time went on, I think Universal just started focusing their efforts on movies and franchises other than LBT. Who knows...maybe with all the sequel naysayers out there, they felt like their efforts to continue the series were futile now that it was heading into double-digit numbers.