In analyzing the problematic plots, I found it helpful to contrast with a book that did succeed where the others failed. Because book three of the library haul was a winner: Drums, Girls and Dangerous Pie by Jordan Sonnenblick. For brevity, I'll cite it as DG&DP.
The best books' subplots relate to the main plot. They complicate matters or shed light on the core story problem or bring surprising help to the MC. Less effective subplots pop in to create tension for the sake of tension, feel inorganic and almost never resolve.
I see three core areas to address in your plot when thinking about and creating effective subplots.
StakesIf your MC's dilemma is too low-stakes, you will be tempted to create tension for tension's sake with very random plot elements. You know what I mean--explosions, zombie attacks, unmotivated fights, suicide attempts, unplanned pregnancies and the like--because something needs to happen here.
Your novel's protagonist must have something at stake worthy of a 200+ page exploration. There must be something of great value to be lost, and the cost of that loss should be devastating. One can raise the stakes over the course of the book by upping the value of the desired thing (make winning it have multiple rewards) and/or making failure appear more and more costly (make losing it have multiple punishing effects).
DG&DP opens with 13-yo Steven voicing self-confidence problems and resentment of his angelic little brother. When angelic brother is diagnosed with leukemia, Steven's issues take on a whole new twist. Fighting for attention from parents and seeking status at school continue to plague Steven, and small wins in these subplots help him cope with the high-stakes main plot. It's because Steven is dealing with such a huge issue--possible death of a family member--that his moments of struggle with smallness matter very much. Will this trial make him grow up or regress? If he regresses, will it tear his fragile family apart?
The story-problem stakes give weight to the subplots and the subplots up the stakes of the story problem.
Natural consequencesThe anxiety, stress, or time-suck of your main story problem will cause ripples in other areas of the MC's life. Think through what those might be and you can have tension-building subplots that feel organic.
In DG&DP, Steven's ability to concentrate at school evaporates. He begins to have confrontations with teachers about it. His options for repairing the problem involve tutoring with guess who? That's right, two love interests.
Steven's usual coping mechanism is to escape into music making. As a result, his skill improves and becomes his means of reaching out to others.
Relational falloutIn times of high stress, relationships around your MC will always be tested. Exploring how those around the MC help and/or hinder her can be a great way to build up and release tension. Those she seeks for support may disappoint, distract, disappear. The weak sidekick may show surprising strength when put to the test.
In DG&DP, the father character emotionally shuts down. Steven struggles with the very same tendencies, and seeing how his father's unavailability hurts him, he begins to change.
What are your thoughts about causes of subplot sputter-out? Read any good books lately that have organic subplots that support the main plot and resolve adequately?
A side observation: Sonnenblick's wonderful book was a debut novel, while the unnamed not-so-effective novels were by established authors. I often wonder if established authors are forced to write under tremendous deadline pressure and if their storytelling suffers as a result. What do you think?