Hi Bdaniel, could you post the 3DM model file with your problem curve in it as an attachment? Being able to examine your actual curve would help give you a better answer.

But in order for Construct > Planar to work, the curve needs to be closed and also planar.

You can tell if it's closed by selecting it and then looking a the object type indicator in the properties panel, in the upper-right corner of the main window. If you have a closed curve the object type will read as "Closed crv", and if it's an open curve it will just say "Curve":





If you have more than one curve then you might want to use Edit > Join to join them together so that you can then see if they join into a closed curve or not.

If you have any opening between the start and end of the curve it will prevent planar from being able to work.

- Michael