If the crane on a M1089 wrecker can hoist up but not lower, it may indicate a malfunction in the hydraulic system, such as a faulty control valve or a hydraulic fluid leak. Additionally, the presence of air in the hydraulic lines can cause inconsistent operation. Another possibility is a mechanical blockage or a failure in the winch or cable assembly preventing it from lowering. Inspecting the hydraulic system and components for damage or wear would be essential for troubleshooting the issue.