At a strategic impasse Is there a general guideline for how to parlay a lead in development and a slight positional advantage into a definitive win? I'm playing a game now online, and I'm stuck for a way to keep and exploit the upper hand. I get stuck like this a lot, and I figure it must be for lack of principles or tactical vision.
Converting advantages such as a lead in development or a better position are practices that operate on a higher level (conceptually, at least) than tactics. The most common forms of tactics are those than result in the gain of material or a mating net, much more tangible results than say a weak square or an initiative. Pure calculation and a sharp tactical sense are important aspects of chess, but for the majority of the game they will be largely separate from positional play. Therefore, I would say that this is more supportive of your statement that it is a lack of understanding of principles rather than a weak tactical vision.

Let's address the specific examples that you outlined in your question. First, what does a lead in development entail? There is one main principle that you can utilize - initiative. The initiative is dictating the flow of the game. You can force your opponent to respond to your moves, allowing you to execute whatever plan you want while placing his own goals on hold. In such, it is common for positions where one side has the initiative for that player to launch an attack, given that the opponent is not adequately prepared.

If you can elaborate on the "slight positional advantage" (Minor piece difference? Pawn structure [backwards, isolated, doubled, hanging, passed]? Open files? Weak squares? Space? etc.) that you mentioned in the question, I can recommend other plans that are associated with the appropriate imbalances.