Hi James.
There are a couple of different things that could be causing your issue.
When you go into register, click on the advanced tab. Adjust the star detection threshold down to the lowest percentage. This will allow DSS to pick up more stars. It needs a minimum of 8 clean stars to stack on every frame. You said it picks up 57 stars so this may not be the issue but it's worth a try.
The other thing you can do is below the register button there is one that says compute offsets. Click this and it will register all of your frames and based on the quality of each frame, it will give each frame a score. The highest scoring frame will become your reference frame by which each following frame is stacked. This is where check above threshold comes in. You pick what the lowest score you want to use is and it will ignore everything lower. You may find when you do this that DSS will not come up with a score or calculate the offsets and rotation angles for some frames. This is because the quality of the stars is not good enough to be able to align it. If that is the case, then it's back outside to have another crack.
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I hope this helps