The most common mistakes are avoidable and cheap to fix: an incomplete business description, outdated hours or phone number, reviews and questions left unanswered for weeks, no regular posts, missing or stale photos, and — probably the biggest one — the wrong primary category. Your primary category is one of the strongest signals Google uses to decide which searches you show up for, so a general contractor mis-categorized as a "handyman service" is quietly losing search visibility every day.
None of these require a big budget. They require someone checking the profile every week instead of once at setup and never again.
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Full breakdown: Google Business Profile Optimization for Contractors