I encountered this issue some times with some unofficial parts or real custom parts.
What I've done to fix it ( with the help of Trevor) :
1) Create a "custom" directory in your unofficial directory
2) Copy the parts you need in this directory
3) Launch LPub3D
4) Go in the "Configuration" menu, and choose "Preferences", then the "Other" Tab. In the bottom of the windows, add your new "custom" directory in the list
5) Restart LPub3D
I works for me.
What I've done to fix it ( with the help of Trevor) :
1) Create a "custom" directory in your unofficial directory
2) Copy the parts you need in this directory
3) Launch LPub3D
4) Go in the "Configuration" menu, and choose "Preferences", then the "Other" Tab. In the bottom of the windows, add your new "custom" directory in the list
5) Restart LPub3D
I works for me.