Composing Polygons Causes Line to Disappear

[Matter.Memory]

Hello,

 

Maybe someone else has had this problem:

I am combining two townships' contour lines into one so that I can build a site model, they are 2' contour lines obtained from the state GIS website (all of these lines are polygons). 

My process is to select a few contours, go up to Edit - Select Connected Objects, then Modify - Compose. 

 

This process worked fine for the first 40 or so contour lines but now when I do that, the line disappears, sometimes partially, often entirely.

 

Where are these lines going and what am I doing wrong?

I have uploaded the file for reference.

 

Thank you,

C

 

CSG Solar Array.vwx

[Matter.Memory]

Something which seems to be working is to convert all lines to Polygons (some of the smaller lines are polylines), then use the drafting aid tool "Simplify Polys..." to bring the complexity of the lines down somewhat.