Contractors who cannot access a construction position on a lower floor no longer attempts to complete the construction from directly above

i'm trying to build service road underground (underneath the de-icing pad, see bottom right of screenshot). the material gets delivered and the workers walk to the boxes but then don't pick them up and just stand there. after a while some of the underground road pieces just magically get build.

====================
COLLECTED HARDWARE DATA
====================

Application version: Beta 5.0-2

Device
=====

Model: MacBookPro11,3

Operating system: Mac OS X 10.13.6

RAM size: 16 GB

Battery status: Full
Battery level: 1

Processor device
============

Type: Intel(R) Core(TM) i7-4980HQ CPU @ 2.80GHz
Core count: 8
Frequency: 2 GHz

Grahpics device
===========

Vendor: Apple
Type: NVIDIA GeForce GT 750M
Memory size: 2 GB

Screen summary
===========

Full screen: True
Resolution: 2880 x 1800 @ 60Hz