Why trades quotes go cold

Why do customers not reply to quotes?

Usually because the job is not urgent and deciding takes effort. Silence is far more often procrastination than rejection, especially on planned work like a bathroom or a boiler replacement.

When should I follow up?

Two or three days after sending, and again about a week later. Two touches captures most of what is recoverable without becoming a nuisance.

Reference the specific job rather than sending a generic nudge. A message that names their property and restates the number reads as service; one that reads like a template reads like a mailing list.

What do I do about the ones that stay quiet?

Ask one direct question that makes a no easy to give. An answer either way is worth more than an open file you keep half-thinking about.