There are a few things that could explain this:

1. Thor is able to use his powers against Hela, but they are still new to him and he is not use to wielding them without a "focus" like his hammer. While he is incredibly powerful, he struggles to use his powers effectively.

2. Thanos is no slouch. He's referred to as "the strongest being in the universe" multiple times, and with good reason. Thanos and company have already breached the ship and slaughtered quite a few as guardian warriors (notice the bodies strewn around in the opening scene).

3. Heimdall is similarly incapacitated, despite being a mighty warrior as well.

4. Maw binds Thor after he takes a beating from Thanos

There are a few things that could explain this:

1. Thor is able to use his powers against Hela, but they are still new to him and he is not use to wielding them without a "focus" like his hammer. While he is incredibly powerful, he struggles to use his powers effectively.

2. Thanos is no slouch. He's referred to as the strongest being in the universe" multiple times, and with good reason. Thanos and company have already breached the ship and slaughtered quite a few Asgardian warriors (notice the bodies strewn around in the opening scene).

3. Heimdall is similarly incapacitated, despite being a mighty warrior as well.

4. Ebony Maw binds Thor after he takes a beating from Thanos