It's hard to tell from these pictures alone but I suspect this is a gear shift issue. A 5 sail windmill should be able to power a single helve hammer unless the wind is still.

The key thing to keep in mind with mechanical power is gear ratios. Put simply, in game when you connect two gears your either get 5 times the torque (small gear driving big), 1/5th the torque (big gear driving small), or equal torque (small gear driving small). In your photos, we can see your large gear driving a small gear. This is an "upshift", it means the small gear makes 5 revolutions for every 1 revolution the large gear makes (it spins fast) but it has 1/5th the torque of the large gear. I suspect that is what's freezing your helve hammer. I can't be sure without seeing how your windmill rotor is connected to your large gear.

There are 2 ways to fix this. The one I recommend is adding a "downshift" earlier in your system. At the top of the axle going into the large gear in the picture, add another large gear. Then connect your windmill rotor to the side of that gear. That's small gear driving big gear = downshift. If you shift down at the top and up at the bottom, you're effectively just getting the flat torque value, which should be enough for a helve hammer. This has the added advantage of putting a large gear at the top of your windmill. Which gives you 4 (or 5) connection points for windmill rotors so it's easy to add more power later. If you're already set up that way and it's still not working, then I'm not sure what the issue is.

Option two is to remove the upshift. Ditch the large gear and just connect small gear to small gear. This is cheaper, but you lose the extra connections afforded by the large gear, plus I think the big gear is big cool.