Assets won't snap together - Red connector.
Try holding down F8. Factory assets will occasionally fail to snap together with the snap glyph displaying as red instead of yellow. The asset may be snapping to the floor first, and then seeing a conflict (perhaps only fractions of a millimetre) when trying to also snap to another asset. Holding down F8 temporarily suspends the floor snap, so the connector snap takes precedence and the asset should snap into place as expected."