There is a piece of vital information that would clear up a character's problems, whether it is a misunderstanding of romance or combat. If the character would just spit it out already, it would save entire episodes', if not entire seasons' worth of trouble and tears.
...so, naturally, they don't.
Whether it's due to embarrassment, ego, or just plain stupidity, they rush into whatever situation is going on. Sometimes this leads to a moral about the benefits of clear-headed conversation over fighting needlessly. However, most of the time it's just to provide padding to the series.
Sometimes, one reason for this is because the other side won't let them, or else they seem to have a wall around their mind against the information.
Other times the character is just an ass. Although the Ineffectual Loner has many chances to clarify his role, most of the time he'll just tell the heroes to figure it out themselves.
Another case is where a problem that would be easy to talk through has gotten mixed up in something they're less willing (or strictly unable) to share. It could be a Dark Secret, the Masquerade, a Secret Identity, or even something mundane — but explaining the easy problem means explaining the hidden one, so the ignorant side must be kept in the dark on both, whether the knowledgeable side likes it or not. This tends to be how the Masquerade starts to kill your dating life... or worse, kill the date themselves.
Of course, any time someone finally summons up the courage to actually say it, they will be interrupted. It could be sudden events in the plot taking priority, or another of their circle of friends suddenly feels the need to announce some unimportant item that needs everyone's attention right now. The moment is lost, and even when it isn't, they likely won't be believed anyway. Sometimes, most tragically of all, the character or the person to whom this needs to be said will die or be otherwise lost forever just as the character has gotten up the courage to make his or her feelings known.
Thinking you have courage enough and finding yourself indulging in Talk About the Weather or other chitchat is also common.
Sometimes overcome by the Green-Eyed Monster, or by Converse with the Unconscious or (worst case) Talking to the Dead.
If a character derives pleasure from withholding information and puzzling others, it is Loves Secrecy.
If it's a magical curse that prevents the character from sharing information, then that character may be Tongue-Tied. If it's simply too personal for the character to talk about, they're Emotionally Tongue-Tied. If both members of a romantic pairing can't spit it out, they're Twice Shy. Should the character find saying whatever it is so alien and repugnant that they are practically incapable of saying it, it's probably Gagging on Your Words. If they're too damn mad to get the words out, you're dealing with a case of Angrish. If the character awkwardly manages something vageuly resembling a confession of love that nevertheless gets the message across, it's I "Uh" You, Too.
In an Unresolved Sexual Tension or Belligerent Sexual Tension situation, or if All Love Is Unrequited, this trope is often vital to maintaining the status quo.
Real Life Cannot Spit It Out is known as Alexithymia, which is an inability to describe or process emotions. Sometimes a trait of introverted people whether or not they have this condition.
See also Aborted Declaration of Love, Interrupted Declaration of Love, You Didn't Ask, Mistaken for Index, Hint Dropping, Snowball Lie. A specific sub-trope of Poor Communication Kills. Sister trope of Tongue Twister. Has nothing to do with being unable to clear one's mouth of a disgusting flavor. Everyone Can See It is what happens when the information is romantically related and blatantly obvious to everyone except those who can't spit it out.
