It's hanging up on something, either corrosion build-up or something is bent. My Black Widow front end did the same thing. In my case, corrosion and pitting on both the upper and lower fork was preventing it from rebounding, and lubrication barely made an improvement. Are you using new bushings? They're available from Black Widow Motorsports. Avoid re-using old, brittle bushings. You may need to sand down any pitting with some emery cloth, clean it and re-grease. Both upper and lower forks need to be super smooth and clean.
furyus