Most marketing agencies focus on bringing new buyers to your website. That matters, but for most manufacturers a large share of revenue comes through people who already know you: distributors, reps, and existing customers. Agencies rarely build anything for those channels, because it doesn't show up in their traffic reports.
When manufacturers try to fix it themselves, the work usually lands on someone who already has a full-time job. A line card gets updated once, a new brochure gets printed for a trade show, and then nothing happens until the next show. Distributor and sales support needs to run as a program, with a plan for each channel, or it doesn't happen at all.