There's no way to build a perfect team... This age more than ever, there's so many possibilities for Pokemon's moveset to be that you can never be too sure.
It definitely isn't like in RBY when every team had a Gyarados and a Dragonite...
Your possibly a 'few dollars short' either because you don't have the best IVs, you aren't using the best EV spread, or if you're not using the best method of battle. IE: Any idiot can use a Blissey. It takes a veteran player to know when to switch into one and how to keep it alive.
It doesn't always mean that your team is lacking something or isn't built right. It may have kinks, like some teams may have a Gyarados weakness, where Gyarados can safely come in on half their team and set up, etc.
My best advice is that you should go to the RMT (Rate My Team) sub forum here and post your usual team, along with their EVs, IVs, movesets, etc. You'll get the best feedback there on how to improve your team.