We see this quite often on the Tech Support line - people having trouble trying to get sweeps to work.
Here is a typical example: -
It looks relatively simple - the Path on the left and the angled Guide Curve on the right have both been created before the Profile sketch, but there are issues when we try to make the feature...
It's all going swimmingly, until we try and put in the Guide Curve...
The the preview disappears - which (generally) means it's not going to work.
If we edit the Profile sketch...
We can see that in this case there is a coincident relation between the top right corner of the profile and the right edge of the part. This is stopping the profile from following along the guide curve.
But hang on - the profile also has coincident relations to the ends of the Path and the Guide Curves -
these can also cause issues. What we should have are Pierce Relations.
To make a Pierce sketch relation, you select the point in your Profile sketch (remember, you made that after your Path and Guide Curves didn't you?) and the line of the guide curve itself - not the end of it.
As a general rule, I try and avoid any relations from the Profile to anything other than the Path and the Guide Curves - and only use Pierce relations to those.
If you do that, life will be easier!