In the attached image, I could not 3D Add the cyan and green ACIS Solids. I tried minor adjustments (Quick Pull on different facets, slight movements, etc.), all to no avail. The cyan object was in two pieces before I added them, so I was pretty certain it was the green one causing the problem. I confirmed this when I could not 3D Add a Box to the green object.
So, I selected the green object and Save As STEP. I then opened the STEP file in a new window and copied it over to the original window. After aligning the two, 3D Add worked easily.
The only drawback (and possible problem later) is that the file size jumped from 1.1 MB to 6.6 MB. I am wondering if I will have any problems when I eventually export to STL.
TC Pro Platinum 2019, 2018, 2017, 2016 & 2015 (all with LightWorks & RedSDK) & V21
System: i7-5820K @ 3.30GHz, ASRock X99 Extreme4, 16GB DDR4-2133 RAM, Gigabyte GTX 970, Samsung NVMe SSD 950 (256GB), Windows 7 Pro (64-bit) SP1