Is the tiller stiff and resistant at the dock? Not sure what you mean by 'heavy' at the dock.
Your bushings could be dirty/somehow siezing the tiller. Some people make new nylon bushings but plenty of plastics swell when wet, leading to binding and difficulty steering. (can be a built-in tiller lock, though!
- but overall it's a pain)
No reason for a tiller not to turn freely at the dock unless it's a bushing/sleeve issue.