Hi @victorwongiam,
Hope you are well! 
Is it only Vertical Conduits, through the slab without any bends? If so, you can also make use of the ARRAY TOOL and create a linear array.
Also, if you have bends and you need to use the parallel conduits tool, I would select the runs in the elevation/section, then click on my plan view window to activate it, then use the parallel conduits tool from there.
Let me know if you still receive the same problem. 
Rudi Roux
MSc | Digital Engineering Manager
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Revit Mechanical & Electrical Systems 2018 Certified Professional | Revit MEP & Architecture 2015 Certified Professional
AutoCAD 2015 Certified Professional | Autodesk Building Performance Analysis (BPA) Certificate
If this post resolved your issue, kindly Accept as the Solution below. Kudos are always welcome ⇘