Hi Alex
Meshes do not take part of the Priority Based Connections. So you actually have to connect the Mesh to the slab manually (right-click / Connect ...) and using the Solid Element Operations Panel.
Typically you will make the slab "dig" into the Mesh by Subtracting with upward Extrusion.
Check it here
http://helpcenter.graphisoft.com/guides/archicad-17-int-reference-guide/virtual-building/element-extras-trussmaker-solid-element-operations/solid-element-operations/solid-operation-types/
Hope it helps
Nando Mogollon
Director @ BuilDigital
nando@buildigital.com.au
Using, Archicad Latest AU and INT. Revit Latest (have to keep comparing notes)
More and more... IFC.js, IFCOpenShell
All things Solibri and BIMCollab