There are... quite a few, actually.
Animal Crossing is a series I used to love, but the games have just felt like more of the same every time. I get bored of them much more quickly now.
I absolutely loved the first three Mario & Luigi games, but Dream Team started to peter out towards the end for me. I didn't buy Paper Jam until it was on sale almost a year after it came out, and dropped it very quickly. May not bother with the next one at all.
Paper Mario is another series I used to adore. The direction the newer games have gone is not something I really want out of the series, though. I dropped Sticker Star pretty quickly, and never bothered with Color Splash.
Halo and Gears of War are series that I used to play a lot of, but steadily played less and less of each iteration.
Uncharted is another good one. Loved the first two, still liked the third okay, but by the fourth game I was starting to get tired of the formula. Never finished Lost Legacy, and probably won't bother with the series anymore after this, if it continues.
And then there's Fire Emblem, which hasn't quite reached this point yet, but I'm definitely less enthusiastic about it than I used to be.