I’ve had the exact same frustration, finding store bought adventures typically sloppy or incomplete. Publishers seem more interested in releasing rulebooks and supplements than creating solid adventures. Some of their reasoning might be that many GM’s prefer thin adventures to support improvisation. Since you’re asking this question, I assume you’re like me and prefer something more planned.
Improvisation vs. Planning
Strictly speaking improvisation is no less valid than planning. Both have advantages and disadvantages. Improvisation is faster, a real advantage for GM’s considering how much work they must do. Improvisation also helps keep play connected to player interests. Planned adventures are more consistent and more focused, key to keeping a gaming session connected to the greater campaign. The best gameplay employs elements of both, but as you’ve noticed: Store bought adventures favor improvisation.
Plugging an Adventure into Your Campaign
What the players do every session constitutes an adventure, essentially a chapter in the greater story of the campaign. No matter how complete a store-bought adventure is, it can never connect to your greater campaign without modification. So I find some amount of tweaking is always needed. Deciding how the adventure slots into your campaign is good place to start. Once you understand it’s place in your campaign, it should prove easier to modify the adventure further, or you may find that you can improvise from there.
Tweaking Adventures Further
It’s often easier to notice that something is wrong with a store-bought adventure than to understand how to fix it. Below are some suggestions for further tweaking.
Like a narrative, much of RPG gaming consists of plot - the stuff that’s happening. Plot is a chain of cause and effect. Because Nick Fury sent players to Antarctica, the villains there respond by doing A, which in turn causes the players to do B, and so on. When reviewing a store bought adventure, confirm that the events obey a proper cause and effect structure, particularly at the adventure’s beginning.
Every adventure will have at the least one character: an antagonist; a monster or villain opposing the player’s objectives. Many adventures fail to provide an actual conflict between the PC’s and the antagonist. Conflict is based on divergent goals, the players want A, but the villain wants B. They can’t both get what they want, thus the conflict. For clarity, stopping the bad guys is not a goal, nor is having fun playing. Advancement within the SHIELD organization (requiring the successful completion of the Antarctic mission) is a goal. Many store-bought adventures use alignment as a shorthand. The antagonist opposes the players because they are evil, but evil itself is not a goal. Review the material and confirm that the other characters truly have adventure goals that conflict with the PC’s campaign goals.
When gaming sessions take on the quality of storytelling, players find them far more engaging. Simply put, story connects plot to character, specifically the PC’s. Adventures that merely inflict events (plot) on the PC's, forcing them to react, fail to rise to the level of story. For that, an adventure must allow players to make decisions that cause the antagonists to react. This is ‘character driven’ storytelling. Many store-bought adventures are organized as set pieces, being map-based. Challenges reside at locations waiting for PC’s to arrive and trigger them like traps. If events are provided, they take the form of random encounters of little consequence to the adventure’s plot. Devise major events driven by likely player decisions, that will drive the plot and give the map a supporting rather than starring role.