When I try selecting a 3D Objects through mouse left-click, it doesn't always select the object and/

This is a common issue when working with geolocated models in version SYNCHRO Pro v6.2.2.0 and prior. You can check if the model is very far from the project base coordinate (0,0,0) by selecting an object and clicking on Get Bounding Box from under 3D Properties -

If it's a large value, i.e., has values with "e" in it, then it's far from the base coordinates. The recommended workflow is to relocate the project closer to the project base coordinate (0,0,0) within the original modeling software, before exporting it out.

Alternatively, use SYNCHRO Control to create iModels using iTwin Connectors. You can learn more about this here -