Hello,

Please check the two option below,

1. When you are connecting your elbow with straight pieces, make sure that the small arrow of attacher is showing green to drop it downwards. If it is showing blue that means its going to upward and it will not show hidden untill you close upward face by an end cap or another elbow.

2. Kindly check it once again and if the problem is still there then, probably you have to check LTS of the drawing.

Please check and revert your observations.

Thank you.