When Matchmaker is initially configured, you may run into an issue where no leads are being matched at all. A Match Request error similar to the following will be thrown:
DUPLICATES_DETECTED, Use one of these records?
When this error is happening, please check the setup of the duplicate rule and reconcile it with the instructions under "Step 2: Duplicate Management" of the Install & Prepare guide. The problem is likely that the duplicate rule is not set to "Allow" on both Action On Create & Action On Edit and/or that the Alert checkbox is selected.