Digimon Frontier/Tamers
I must admit I was one of the few people who felt Frontier had a promising premise. It started out well and entertaining enough... until about halfway in and it became the Takuya and Kouji show. Junpei, Tomoki, and Izumi did nothing aside of giving their spirits to the two main players and sit back and let them do all the work.
Tamers also had this problem.. Hirokazu and Kenta felt really tacked on, as did Shuichon Ryou pops up out of no where and takes priority over all of them, but even he gets shuffled out (along with Lee and Ruki) later on when it's basicakky Takato's time to shine for the final stretch of episodes. Main offender is Juri: once Leomon died she... litterally did nothing. She went from a girl who wanted to get stronger and help out to damsel in distress who only got captured and sat around moping.
In Adventures the side kids at least god to help out and get their own little storylines to develop somewhat . Tamers and Frontier ignored their side characters, though Frontier to a much harsher degree.
Avatar the Last Airbender
Just about anyone not named Aang or Zuko get shafted. By the third season this becomes even more prevalent. Toph's only real spotlight in the third season is a filler episode (The Runaway), Sokka's character as a whole had been inconsistent throughout the show (Is he the normal smart guy or the dumb, sarcastic, goofy comic relief? It changes per episode) and his sword training goes nowhere; they even throw his special sword away in the finale, (as if he even used the thing much anyway). Mai and Ty Lee also suffer from this. They're pretty much "those two girls who hang out with Azula, you know the ones" for the second season, and in the third they... get a three minute speech about their characters in The Beach and then later get arrested and thrown in jail for the rest of the series so they don't do anything else. The only side character who did anything somewhat worth of merit was Katara, but her character started to really degrade into Wanda-esque levels of naggyness in the third season that I found it hard to care about her at all.
Naruto
Naruto and Sasuke get all the attention, and the rest of the kids get to sit on the side lines and maybe have a random battle every 80 episodes, if they're lucky. Which is a shame considering the ones I find the most interesting (Shino and Chouji) basically are left out in the dust and never used. I actually looked forward to those filler episodes because it meant they would be used.. even if poorly. It was annoying how Naruto had to save the day all the time, but at least they got a bit of spot light.
Basically, any show that has a group of people, but only choose to focus on one of two members of the group despite them having plenty of options.